










                            GLISP USER'S MANUAL
                            GLISP USER'S MANUAL
                            GLISP USER'S MANUAL






                            Gordon S. Novak Jr.
                        Computer Science Department
                            Stanford University
                        Stanford, California  94305







                                   DRAFT
                                   DRAFT
                                   DRAFT






                             17 February 1982











This  research was supported by NSF grant SED-7912803 in the Joint National
Science Foundation - National Institute of Education Program of Research on
Cognitive  Processes  and  the  Structure  of  Knowledge  in  Science   and
Mathematics.
                                                                          1


                                 CHAPTER 1
                                 CHAPTER 1
                                 CHAPTER 1

                               INTRODUCTION
                               INTRODUCTION
                               INTRODUCTION



1.1. Overview of GLISP
1.1. Overview of GLISP
1.1. Overview of GLISP

  GLISP  is  a  LISP-based  language  which  provides  high-level  language

features not found in ordinary LISP.  The GLISP language is implemented  by

means of a compiler which accepts GLISP as input and produces ordinary LISP

as  output; this output can be further compiled to machine code by the LISP

compiler.



  The goal of GLISP is to allow structured objects to be  referenced  in  a

convenient, succinct language, and to allow the structures of objects to be

changed without changing the code which references the objects.  The syntax

of  many GLISP constructs is English-like; much of the power and brevity of

GLISP derive from the compiler features necessary to support the relatively

informal, English-like language constructs.  The following example function

illustrates how GLISP permits definite reference to structured objects.



    (HourlySalaries (GLAMBDA ( (a DEPARTMENT) )
       (for each EMPLOYEE who is HOURLY
          (PRIN1 NAME) (SPACES 3) (PRINT SALARY) )  ))



The features provided by GLISP include the following:


   1. GLISP maintains knowledge of the "context" of the computation as
      the program is executed.   Features  of  objects  which  are  in
      context  may be referenced directly; the compiler will determine
      how to reference the objects given the current context, and will
      add the newly referenced objects to the context.  In  the  above
      example,  the  function's  argument,  an  object  whose class is
      DEPARTMENT, establishes an initial  context  relative  to  which
                                                                          2


      EMPLOYEEs can be found.  In the context of an EMPLOYEE, NAME and
      SALARY can be found.

   2. GLISP  supports  flexible object definition and reference with a
      powerful abstract datatype facility.  Object classes are  easily
      declared  to  the  system.    An  object  declaration includes a
      definition  of  the  storage  structure  of   the   object   and
      declarations  of properties of the object; these may be declared
      in such a way that they compile  open,  resulting  in  efficient
      object  code.    GLISP  supports object-centered programming, in
      which processes are invoked  by  means  of  "messages"  sent  to
      objects.    Object  structures may be LISP structures (for which
      code is automatically compiled) or Units in the user's  favorite
      representation   language   (for   which  the  user  can  supply
      compilation functions).

   3. Loop constructs, such as  (FOR EACH <item>  WITH  <property>  DO
      ...) , are compiled into loops of the appropriate form.

   4. Compilation  of infix expressions is provided for the arithmetic
      operators and for additional  operators  which  facilitate  list
      manipulation.   Operator overloading for user-defined objects is
      provided using the message facility.

   5. The GLISP compiler infers the types of  objects  when  possible,
      and  uses  this knowledge to generate efficient object code.  By
                  ___________ ________ __ _ _________ ____
      performing  compilation relative to a knowledge base , GLISP  is
      able  to  perform  certain computations (e.g., inheritance of an
      attached procedure from  a  parent  class  of  an  object  in  a
      knowledge   base)  at  compile  time  rather  than  at  runtime,
      resulting in much faster execution.

   6. By separating object definitions from the code which  references
      objects, GLISP permits radical changes to object structures with
      no changes to code _ a goal long sought in high-level languages,
      but  one  which  has  largely  been  unrealized  for  structures
      involving pointers.



1.2. Implementation
1.2. Implementation
1.2. Implementation

  GLISP is implemented by means of a compiler, which incrementally compiles

each function  the  first  time  the  function  is  called.    [Of  course,

compilation  can  be  invoked  directly  as  well.]   GLISP  functions  are

indicated by  the  use  of  GLAMBDA  instead  of  LAMBDA  in  the  function

definition.  When the INTERLISP interpreter sees the GLAMBDA, it effects an
                                                                          3


                                  1
"interrupt"  to the GLISP compiler  , which compiles the GLISP function and

returns a normal LISP EXPR; thereafter, the LISP version is  used.    Thus,

use  of  GLISP  entails  the cost of a single compilation, but otherwise is

about as efficient as normal LISP.  The LISP code produced by GLISP can  be

further compiled to machine code by the LISP compiler.



  To   use   GLISP,   it   is   only   necessary   to  load  the  compiler:

   LOAD(GLISP.LSP)
   LOAD(GLISP.LSP)
   LOAD(GLISP.LSP)  .  Thereafter, whenever a function which has GLAMBDA in

its definition is interpreted, the compiler will be  called  automatically.

The  GLISP compiler is also called automatically when LISP compilation of a

function is requested.  An individual function can be  compiled  explicitly

by invoking GLCOMPILE(<fn>), where <fn> is the name of the function.  If it

is  desired  to  explicitly compile all the GLISP functions in a file, this

can be done by invoking (GLCOMPCOMS <file>COMS), where <file>COMS is  bound

to the list of file package commands for the file; this will call the GLISP

compiler for each function whose definition begins with GLAMBDA.



  The  compiled code, result type, and original code for compiled functions

are stored on the property list of the function name.  Properties of  GLISP

functions   and   Structure   names  can  be  examined  with  the  function

GLED(<name>), which calls the INTERLISP editor  on  the  property  list  of

<name>.




_______________

  1
   using the LAMBDATRAN feature of INTERLISP, written by Ron Kaplan.
                                                                          4


                                 CHAPTER 2
                                 CHAPTER 2
                                 CHAPTER 2

                            OBJECT DESCRIPTIONS
                            OBJECT DESCRIPTIONS
                            OBJECT DESCRIPTIONS



2.1. Declaration of Object Descriptions
2.1. Declaration of Object Descriptions
2.1. Declaration of Object Descriptions

      ______  ___________
  An  Object  Description  in GLISP is a description of the structure of an

object in terms of named substructures, together with definitions  of  ways

                                                      _______ ______
of  referencing  the  object.  The latter may include virtual fields (i.e.,

fields whose values are not stored, but are computed  from  the  values  of

                                              ________
other  fields),  adjectival  predicates,  and messages which the object can

receive; the messages can be used to  implement  operator  overloading  and

other compilation features.



  Object Descriptions are obtained by GLISP in several ways:


   1. The   descriptions   of  basic  datatypes  (e.g.,  INTEGER)  are
      automatically known to the compiler.

   2. Structure descriptions (but not full object descriptions) may be
              _____
      used as types in function definitions.

   3. The user may declare object descriptions to the system using the
      function GLDEFSTRQ.

   4. Object descriptions may be  included  as  part  of  a  knowledge
      representation  language, and are then furnished to GLISP by the
      interface package written for that representation language.



  LISP data structures are declared using the  function  GLDEFSTRQ  ("GLisp

DEFine  STRucture Quote").  GLDEFSTRQ takes one or more object descriptions

as arguments, assuming the descriptions to be quoted.  The format  of  each

description is as follows:
                                                                          5



    (<object-name>  <structure-description>
              PROP  <property-descriptions>
              ADJ   <adjective-descriptions>
              ISA   <predicate-descriptions>
              MSG   <message-descriptions>   )



The  <object-name>  and  <structure-description>  are  required;  the other

property/value pairs are optional, and may  appear  in  any  order.    Each

<description>  specified  with  PROP,  ADJ,  ISA,  or MSG has the following

format:


    (<name> <response> <prop > <value > ... <prop > <value >)
                            1        1           n        n

where <name> is the (atomic) name of the property, <response> is a function

name or a list of GLISP code to be compiled in place of the  property,  and

the  <prop><value>  pairs are optional properties which affect compilation.

The compilation of all of these properties  is  described  in  the  section

"Compilation of Messages".



  Once  declared,  object descriptions may be included in INTERLISP program

files by including in the <file>COMS a statement of the form:


    (GLISPOBJECTS <object-name > ... <object-name >)
                              1                  n


  The following example illustrates some of the declarations which might be

                                 ______
made to describe the object type Vector.
                                                                          6



    (GLDEFSTRQ

       (VECTOR   (CONS (X NUMBER) (Y NUMBER))

          PROP   ( (MAGNITUDE  ((SQRT X*X + Y*Y))) )

          ADJ    ( (ZERO       (X IS ZERO AND Y IS ZERO))
                   (NORMALIZED (MAGNITUDE = 1.0)) )

          MSG    ( (+          VECTORPLUS OPEN T)
                   (-          VECTORDIFFERENCE) )

         ))



Since  GLISP  compilation  is  performed  relative to the knowledge base of

object descriptions, the object descriptions  must  be  declared  prior  to

GLISP compilation of functions using those descriptions.



2.2. Structure Descriptions
2.2. Structure Descriptions
2.2. Structure Descriptions

  Much  of  the  power  of  GLISP  is  derived  from  its  use of Structure

Descriptions.  A Structure Description (abbreviated "<sd>") is a  means  of

describing  a  LISP  data  structure  and  giving  names  to  parts  of the

structure; it is similar in concept to  a  Record  declaration  in  PASCAL.

Structure  descriptions  are used by the GLISP compiler to generate code to

retrieve and store parts of structures.


2.2.1. Syntax of Structure Descriptions
2.2.1. Syntax of Structure Descriptions
2.2.1. Syntax of Structure Descriptions

  The syntax of structure descriptions is recursively defined in  terms  of

basic  types  and composite types which are built up from basic types.  The
                                               2
syntax of structure descriptions is as follows: 
_______________

  2
   The names of the basic types and the structuring operators  must  appear
in  upper-case  as  shown  here.  In general, other GLISP keywords and user
program names may be in upper-case, lower-case, or mixed-case.
                                                                          7


   1. The following basic types are known to the compiler:



      ATOM
      ATOM
      ATOM
      INTEGER
      INTEGER
      INTEGER
      REAL
      REAL
      REAL
      NUMBER
      NUMBER
      NUMBER                (either INTEGER or REAL)
      STRING
      STRING
      STRING
      BOOLEAN
      BOOLEAN
      BOOLEAN               (either T or NIL)
      ANYTHING
      ANYTHING
      ANYTHING              (an arbitrary structure)


   2. An  object  type  which  is known to the compiler, either from a
      GLDEFSTRQ declaration or because it is a Class of units  in  the
      user's  knowledge  representation  language, is a valid type for
      use in a structure description.  The <name>  of such  an  object
      type may be specified directly as <name> or, for readability, as
                                   3
       (A       )      (An       )
       (A       )      (An       )
       (A <name>)  or  (An <name>).

   3. Any substructure can be named by enclosing it in a list prefixed
      by   the   name:    (<name>  <sd>) .     This  allows  the  same
      substructure to have multiple names.  The names used in  forming
      composite types (given below) are treated as reserved words, and
      may not be used as names.

   4. Composite  Structures:   Structured data types composed of other
      structures  are  described  using  the   following   structuring
      operators:


             CONS
             CONS
         a. (CONS <sd > <sd >)
                     1     2
            The  CONS  of  two structures whose descriptions are <sd >
                                                                    1
            and <sd >.
                   2
             LIST
             LIST
         b. (LIST <sd > <sd > ... <sd >)
                     1     2         n
            A list of exactly  the  elements  whose  descriptions  are

            <sd > <sd > ... <sd >.
               1     2         n
             LISTOF
             LISTOF
         c. (LISTOF <sd>)


_______________

  3
                     (A ...)                               (An ...)
                     (A ...)                               (An ...)
   Whenever the form (A ...) is allowed in GLISP, the form (An ...) is also
allowed.
                                                                          8


            A  list  of  zero  or more elements, each of which has the
            description <sd>.

             ALIST
             ALIST
         d. (ALIST (<name > <sd >) ... (<name > <sd >))
                         1     1             n     n
            An association list in which the atom <name >, if present,
                                                       i
            is associated with a structure whose description is <sd >.
                                                                   i
             ATOM    BINDING
             ATOM    BINDING
         e. (ATOM   (BINDING <sd>)

                 PROPLIST
                 PROPLIST
                (PROPLIST (<pname > <sd >) ... (<pname > <sd >) ))
                                 1     1              n     n
            This  describes  an  atom  with  its  binding  and/or  its
            property list; either the BINDING or  the  PROPLIST  group
            may be omitted.  Each property name <pname > is treated as
                                                      i
            a  property  list  indicator  as  well  as the name of the
            substructure.   When  creation  of  such  a  structure  is
            specified,  GLISP  will  compile  code  to create a GENSYM
            atom.

             TRANSPARENT
             TRANSPARENT
         f. (TRANSPARENT <name>)
            An  object  of  type  <name>  is  incorporated  into   the
                                         ___________ ____
            structure  being  defined in transparent mode, which means
            that all fields and  properties  of  the  object  of  type
            <name>   can  be  directly  referenced  as  if  they  were
            properties of the object being defined.    The  object  of
            type  <name>  may  also  contain  TRANSPARENT objects; the
            graph of TRANSPARENT object references must of  course  be
            acyclic.

             RECORD
             RECORD
         g. (RECORD <recordname> (<name ><sd >) ... (<name ><sd >))
                                       1    1             n    n
            This  description  allows  the  use of INTERLISP RECORD or
            DATATYPE records with GLISP.  <recordname> is the name  of
            the  record  type  (which  must  be declared separately to
            INTERLISP); the ordering  of  the  (<name><sd>)  pairs  is
            independent  of the actual structure of the record.  GLISP
                         fetch  replace      create
                         fetch  replace      create
            will compile fetch, replace, and create for access to  and
            creation of RECORD structures.


2.2.2. Examples of Structure Descriptions
2.2.2. Examples of Structure Descriptions
2.2.2. Examples of Structure Descriptions

  The following examples illustrate the use of Structure Descriptions.
                                                                          9



    (GLDEFSTRQ

        (CAT (LIST (NAME ATOM)
                   (PROPERTIES (LIST (CONS (SEX ATOM)
                                           (WEIGHT INTEGER))
                                     (AGE INTEGER)
                                     (COLOR ATOM)))
                   (LIKESCATNIP BOOLEAN)))

        (GRANDMOTHER (ATOM
             (PROPLIST
                  (GRANDCHILDREN (LISTOF (A PERSON)))
                  (AGE INTEGER)
                  (PETS (LIST (CATS (LISTOF (A CAT)))
                              (DOGS (LISTOF (A DOG))) ))
                 )))
       )



The  first  structure,  CAT,  is  entirely  composed of list structure.  An

example CAT might look like:


    (PUFF ((MALE . 10) 5 CALICO) T)


Given a CAT object X, we could ask for its WEIGHT  [equivalent  to  (CDAADR

X)] or for a subrecord such as PROPERTIES [equivalent to (CADR X)].  Having

set a variable Y to the PROPERTIES, we could also ask for the WEIGHT from Y

[equivalent  to  (CDAR  Y)].  In general, whenever a subrecord is accessed,

the structure description of the subrecord is associated  with  it  by  the
                                                                     4
compiler,  enabling further accesses to parts of the subrecord.  Thus , the

meaning of a subrecord name depends on the type of record  from  which  the

subrecord is to be retrieved.  The subrecord AGE has two different meanings

when  applied to GRANDMOTHERs and CATs.  The second structure, GRANDMOTHER,


_______________

  4
   in contrast to the CLISP record package
                                                                         10


illustrates a description of an object which is a LISP atom with properties

stored  on  its  property  list.    Whereas no structure names appear in an

                                             PROPLIST      ALIST
                                             PROPLIST      ALIST
actual CAT structure, the substructures of a PROPLIST (or  ALIST)  operator

must be named, and the names appear in the actual structures.  For example,

if X is a GRANDMOTHER structure, retrieval of the AGE of X is equivalent to

(GETPROP X (QUOTE AGE)).     A  subrecord  of  a  PROPLIST  record  can  be

referenced directly; e.g., one can  ask  for  the  DOGS  of  a  GRANDMOTHER

directly,  without  cognizance  of  the  fact that DOGS is part of the PETS

property.
                                                                         11


                                 CHAPTER 3
                                 CHAPTER 3
                                 CHAPTER 3

                           REFERENCE TO OBJECTS
                           REFERENCE TO OBJECTS
                           REFERENCE TO OBJECTS



3.1. Accessing Objects
3.1. Accessing Objects
3.1. Accessing Objects

  The  problem  of  reference is the problem of determining what object, or

feature of a structured object, is referred to by some part of a  statement

in  a  language.  Most programming languages solve the problem of reference

by unique naming: each distinct object in a program unit has a unique name,

and is referenced by that name.  Reference to a part of a structured object

is done by giving the name of the variable denoting that object and a  path

specification which tells how to get to the desired part from the whole.



  GLISP  permits  reference by unique naming and path specification, but in

                   ________  _________  ________  __  _______      ________
addition  permits  definite  reference  relative  to  context.  A  definite

_________
reference  is  a reference to an object which has not been explicitly named

before, but which can be understood relative  to  the  current  context  of

computation.    If,  for  example,  an  object  of  type VECTOR (as defined

earlier) is in context, the program statement


    (IF X IS NEGATIVE ...


contains a definite reference to "X", which may be  interpreted  as  the  X

substructure  of  the  VECTOR  which  is in context.  The definition of the

computational context and the way in which definite references are resolved

are covered in a later section of this manual.



  In the following section, which describes the syntaxes  of  reference  to
                                                                         12


objects  in  GLISP,  the  following  notation is used.  "<var>" refers to a

variable name in the usual  LISP  sense,  i.e.,  a  LAMBDA  variable,  PROG

variable, or GLOBAL variable; the variable is assumed to point to (be bound

to)  an  object.    "<type>"  refers  to the type of object pointed to by a

variable.  "<property>" refers to a property or subrecord of an object.



  Two syntaxes are available for  reference  to  objects:  an  English-like

syntax,  and a PASCAL-like (or CLISP-like) syntax.  The two are equivalent,

and may be intermixed freely within a GLISP function.  The allowable  forms

of references in the two syntaxes are shown in the table below.


________ ______          _________ ______         _______
"PASCAL" Syntax          "English" Syntax         Meaning

<var>                    <var>                    The object denoted
                                                  by <var>
:                        The
:                        The
:<type>                  The <type>               The object whose type
                                                  is <type>
:                        The
:                        The
:<property>              The <property>           The <property> of
__
or <property>                                     some object
     :                   The            of
     :                   The            of
<var>:<property>         The <property> of <var>  The <property> of the
                                                  object denoted by <var>


These  forms can be extended to specify longer paths in the obvious way, as

in  "The  AGE  of  the  SPOUSE  of  the  HEAD   of   the   DEPARTMENT"   or

"DEPARTMENT:HEAD:SPOUSE:AGE".    Note  that there is no distinction between

reference to substructures and reference to properties as far as the syntax

of the referencing code is concerned; this facilitates hiding the  internal

structures of objects.
                                                                         13


3.2. Creation of Objects
3.2. Creation of Objects
3.2. Creation of Objects

  GLISP allows the creation of structures to be specified by expressions of

the form:


          ____
          ____
          ____
(A        with             =          ,     ,             =         )
(A        with             =          ,     ,             =         )
(A <type> with <property > = <value > , ... , <property > = <value >)
                        1          1                   n          n


                           ____
In  this  expression, the "with", "=", and "," are allowed for readability,

but may be omitted if desired; if present, they must all  be  delimited  on

both  sides  by  blanks.    In  response  to such an expression, GLISP will

generate code to create the specified structure.  The <property> names  may

be  specified  in  any  order.    Unspecified  properties will be defaulted

according to the following rules:


   1. Basic types will be defaulted to 0 for INTEGER and  NUMBER,  0.0
      for REAL, and NIL for other types.

   2. Composite  structures will be created from the defaults of their
      components, except that missing PROPLIST and ALIST items will be
      omitted.


Except for missing PROPLIST and ALIST elements, as  noted  above,  a  newly

created  LISP  structure  will  contain  all of the fields specified in its

structure description.



3.3. Predicates on Objects
3.3. Predicates on Objects
3.3. Predicates on Objects

  Adjectives defined for structures using the ADJ  and  ISA  specifications

                                                     If     For
                                                     If     For
may  be  used in predicate expressions on objects in If and For statements.

The syntax of basic predicate expressions is:


             is
             is
    <object> is <adjective>
             is a
             is a
    <object> is a <isa-adjective>
                                                                         14


Basic  predicate  expressions  may be combined using AND, OR, NOT or ~, and

grouping parentheses.



  The compiler has pre-defined  the  LISP  adjectives  ATOMIC,  NULL,  NIL,

INTEGER,  REAL,  SMALL,  ZERO,  NUMERIC,  NEGATIVE, and MINUS, and the ISA-

adjectives ATOM, LIST, NUMBER, INTEGER, and LITATOM; user definitions  have

precedence over these pre-defined adjectives.


3.3.1. Self-Recognition Adjectives
3.3.1. Self-Recognition Adjectives
3.3.1. Self-Recognition Adjectives

  If  the  ISA-adjective    self  is defined for an object Class, the Class

name may be used as an ISA-adjective to test whether a given  object  is  a

member  of  that Class.  Given a predicate phrase of the form " X is a Y ",

the compiler first looks at the definition of the object class of   X    to

see  if    Y   is defined as an ISA-adjective for such objects.  If no such

ISA-adjective is found, and  Y  is the name of  a  Class  of  objects,  the

compiler looks to see if  self  is defined as an ISA-adjective for  Y , and

if so, compiles it.



                                                                        If
                                                                        If
  If  a    self   ISA-adjective predicate is compiled as the test of an If,

While     For
While     For
While, or For statement, and the tested object is a  simple  variable,  the

variable  will  be  known  to be of that type within the scope of the test.

For example, in the statement



       (If X is a FOO then (_ X Print) ...



the compiler will know that X is a FOO  if  the  test  succeeds,  and  will

compile  the Print message appropriate for a FOO, even if the type of X was
                                                                         15


declared  as  something  other than FOO earlier.  This feature is useful in

implementing disjunctive types, as discussed in a later section.
                                                                         16


                                 CHAPTER 4
                                 CHAPTER 4
                                 CHAPTER 4

                           GLISP PROGRAM SYNTAX
                           GLISP PROGRAM SYNTAX
                           GLISP PROGRAM SYNTAX



4.1. Function Syntax
4.1. Function Syntax
4.1. Function Syntax

  GLISP  function  syntax  is essentially the same as that of LISP with the

addition of type information and RESULT and GLOBAL declarations.  The basic
                   5
function syntax is: 



                      GLAMBDA
                      GLAMBDA
    (<function-name> (GLAMBDA (<arguments>)
                              ______
                              ______
                              ______
                             (RESULT                     )
                             (RESULT                     )
                             (RESULT <result-description>)
                              ______
                              ______
                              ______
                             (GLOBAL                               )
                             (GLOBAL                               )
                             (GLOBAL <global-variable-descriptions>)
          (PROG (<prog-variables>)
                <code>   )))



The RESULT declaration is optional; in many cases, the compiler will  infer

the  result  type automatically.  The main use of the RESULT declaration is

to allow the compiler to determine the result type  without  compiling  the

function,  which  may be useful when compiling another function which calls

it.  The  <result-description>  is  a  standard  structure  description  or

<type>.



  The  GLOBAL  declaration is used to inform the compiler of the <type>s of

any variables used globally; this declaration is optional, but it  must  be

used if subrecords of global variables are to be referenced.




_______________

  5
   The  PROG is not required; RESULT and GLOBAL, if present, must be in the
order shown.
                                                                         17


  The  major  difference between a GLISP function definition and a standard

LISP definition is the presence of type declarations for  variables,  which

are in PASCAL-like syntax of the following forms:



              :
              :
    <variable>:<type>
              :(A       )
              :(A       )
    <variable>:(A <type>)
              ,          ,   :
              ,          ,   :
    <variable>,<variable>,...:<type>
              ,          ,   :(A       )
              ,          ,   :(A       )
    <variable>,<variable>,...:(A <type>)
              :
              :
              :<type>
               (A       )
               (A       )
               (A <type>)



In  addition  to  declared  <type>s,  a  Structure  Description may be used

directly as a <type> in a variable declaration.



  Type declarations are required only for variables whose  subrecords  will

be  referenced.  In general, if the value of a variable is computed in such

a way that the type of the value can be inferred, the variable will receive

the appropriate type automatically; in such cases, no type  declaration  is

                                    _______
necessary.  Since GLISP maintains a context of the computation, it is often

unnecessary  to name a variable which is an argument of a function; in such

cases, it is only necessary to specify the <type> of the argument, as shown

in the latter two syntax forms above.  PROG and  GLOBAL  declarations  must

always  specify  variable  names  (with  optional  types);  the  ability to

directly reference features of objects reduces the number of PROG variables

needed in many cases.  Initial values for PROG variables may be  specified,

as  in INTERLISP; however, the type of a variable which is given an initial

value cannot be explitly specified.
                                                                         18


4.2. Statement Syntax
4.2. Statement Syntax
4.2. Statement Syntax


4.2.1. Expressions
4.2.1. Expressions
4.2.1. Expressions

  GLISP provides translation of infix expressions of the sort usually found

in  programming  languages.   In addition, it provides additional operators

which facilitate list manipulation and other operations.    Overloading  of

                                                                    _______
operators  for  user-defined  types  is  provided  by  means of the message

facility, as described in Section 4.2.3.4.



  Expressions may be written directly in-line within  function  references,

as  in    (SQRT  X*X  +  Y*Y)  , or they may be written within parentheses;

parentheses may be used for grouping in the usual way.   Operators  may  be

                                               ______ ___ ___     ________
written  with  or  without  delimiting spaces, except for the "-" operator,
                                  6
      ____
      ____
      ____
      must
_____ must __ _________ __ ______
which must be delimited by spaces.    Expression  parsing  is  done  by  an

operator  precedence  parser,  using  the  same  precedence  ordering as in
        7
FORTRAN.   The operators which are recognized are as follows:


Assignment               _
Arithmetic               +  -  *  /  ^
Comparison               =  ~=  <  <=  >  >=
Logical                  AND  OR  NOT  ~
Compound                 _+  _-  +_  -_




_______________

  6
   The "-" operator is required to be delimited  by  spaces  since  "-"  is
often  used  as  a  hyphen within variable names.  The "-" operator will be
recognized within "atom" names if the flag GLSEPMINUS is set to T. 

  7
   The precedence of compound operators is higher than assignment but lower
than that of all other operators.  The operators ^ _ _+ +_ _- -_ are right-
associative; all others are left-associative.
                                                                         19


  Each  compound  operator performs an operation involving the arguments of

the operator and assigns a value to the left-hand argument.  The meaning of

a compound operator depends on the type of its left-hand argument, as shown

in the following table:
________       ________       ______         ______         _______
Operator       Mnemonic       NUMBER         LISTOF         BOOLEAN
_+
_+             __________
_+             Accumulate     PLUS           NCONC1         OR
_-
_-             ______
_-             Remove         DIFFERENCE     REMOVE         AND NOT
+_
+_             ____
+_             Push           PLUS           PUSH           OR
                                                8
-_
-_             ___
-_             Pop                           POP


  As an aid in remembering the list operators, the arrow may be thought  of

as representing the list, with the head of the arrow being the front of the

list and the operation (+ or -) appearing where the operation occurs on the

list.   Thus, for example, _+ adds an element at the end of the list, while

+_ adds an element at the front of the list.



  Each of the compound operators performs an assignment  to  its  left-hand

side;  the  above  table  shows  an  abbreviation of the operation which is

performed prior to the assignment.  The following examples show the effects

of the operator "_+" on local variables of different types:


____                     ______ ____              ________ ____
Type                     Source Code              Compiled Code

INTEGER                  I _+ 5                   (SETQ I (IPLUS I 5))
BOOLEAN                  P _+ Q                   (SETQ P (OR P Q))
LISTOF                   L _+ ITEM                (SETQ L (NCONC1 L ITEM))





_______________

  8
   For the Pop operator, the arguments are in  the  reverse  of  the  usual
order,  i.e.,  (TOP -_ STACK) will pop the top element off STACK and assign
the element removed to TOP.
                                                                         20


                                  9
                                  9
                                  9
4.2.1.1. Self-Assignment Operators
4.2.1.1. Self-Assignment Operators
4.2.1.1. Self-Assignment Operators

  There are some cases where it would be desirable to let an object perform

an assignment of its own value.  For example, the user might want to define

____________
PropertyList  as  an  abstract  datatype, with messages such as GETPROP and

PUTPROP,  and  use  PropertyLists  as  substructures  of  other  datatypes.

However,  a  message  such  as PUTPROP may cause the PropertyList object to

modify its own structure, perhaps even changing its structure from NIL to a

non-NIL value.  If the function which implements PUTPROP performs a  normal

assignment  to  its  "self"  variable,  the assignment will affect only the

local variable, and will not  modify  the  PropertyList  component  of  the

containing  structure.   The purpose of the Self-Assignment Operators is to

allow such modification of the value within the containing structure.



  The Self-Assignment Operators are __, __+, _+_, and __-, corresponding to

the operators _, _+, +_, and  _-,  respectively.    The  meaning  of  these

operators  is  that  the assignment is performed to the object on the left-

                            __  ____  ____  ___  _________  __________  ___
hand side of the operator,  as  seen  from  the  structure  containing  the

______
object.



  The  use  of  these  operators  is  highly restricted; any use of a Self-

Assignment Operator must meet all of the following conditions:


   1. A Self-Assignment Operator can only be  used  within  a  Message
      function which is compiled OPEN.


_______________

  9
   This section may be skipped by the casual user of GLISP.
                                                                         21


   2. The  left-hand  side of the assignment must be a simple variable
      which is an argument of the function.

   3. The left-hand-side variable must be  given  a  unique  (unusual)
      name to prevent accidental aliasing with a user variable name.



  As  an  example, the PUTPROP message for a PropertyList datatype could be

implemented as follows:



     (PropertyList.PUTPROP (GLAMBDA (PropertyListPUTPROPself prop val)
          (PropertyListPUTPROPself __
                    (LISTPUT PropertyListPUTPROPself prop val)) ))



4.2.2. Compound Statements
4.2.2. Compound Statements
4.2.2. Compound Statements

  GLISP compiles code for certain  compound  statements  which  allow  more

pleasing syntax than ordinary LISP allows.


4.2.2.1. IF Statement
4.2.2.1. IF Statement
4.2.2.1. IF Statement

  The format of the IF statement is as follows:


                            ____
                            ____
                            ____
     IF                     THEN
     IF                     THEN
    (IF        <condition > THEN <action  > ... <action  >
                         1              11             1i
        ______              ____
        ______              ____
        ______              ____
        ELSEIF              THEN
        ELSEIF              THEN
        ELSEIF <condition > THEN <action  > ... <action  >
                         2              21             2j
        ...
        ____
        ____
        ____
        ELSE
        ELSE
        ELSE   <action  > ... <action  >)
                      m1             mk

Such  a  statement is translated to a COND of the obvious form.  The "THEN"

keyword is optional.
                                                                         22


4.2.2.2. FOR Statement
4.2.2.2. FOR Statement
4.2.2.2. FOR Statement

  The FOR statement generates a loop through a set of elements (typically a

list).  Two syntaxes of the FOR statement are provided:



                    __
                    __
                    __
     FOR EACH       DO
     FOR EACH       DO
    (FOR EACH <set> DO <action > ... <action >)
                              1             n

                             __
                             __
                             __
     FOR            IN       DO
     FOR            IN       DO
    (FOR <variable> IN <set> DO <action > ... <action >)
                                       1             n

The  keyword "DO" is optional.  In the first form of the FOR statement, the

singular form of the <set> is specified; GLISP will convert the  given  set
                          10
name  to  the plural form.    The <set> may be qualified by an adjective or

predicate phrase in  the  first  form;  the  allowable  syntaxes  for  such

qualifying phrases are shown below:


          WITH
          WITH
    <set> WITH <predicate>
          WHICH IS
          WHICH IS
    <set> WHICH IS <adjective>
          WHO IS
          WHO IS
    <set> WHO IS   <adjective>
          THAT IS
          THAT IS
    <set> THAT IS  <adjective>


<predicate>  and <adjective> phrases may be combined with AND, OR, NOT, and

grouping parentheses.  Within the FOR loop, the current member of the <set>

                                                  _______
which is being examined is automatically put into context  at  the  highest

level of priority.



  As  an  example, suppose that the current context contains a substructure

whose description is


_______________

  10
    For names with irregular plurals, the plural form should be put on  the
property  list  of  the singular form under the property name PLURAL, e.g.,
(PUTPROP 'MAN 'PLURAL 'MEN).
                                                                         23


    (PLUMBERS (LISTOF EMPLOYEE))


Assuming  that EMPLOYEE contains the appropriate definitions, the following

FOR loop could be written:


    (for each PLUMBER who is not a TRAINEE do SALARY _+ 1.50)



  To simplify the collection  of  features  of  a  group  of  objects,  the

<action>s in the FOR loop may be replaced by the CLISP-like constructs


              COLLECT
              COLLECT
          ... COLLECT <form>)
              COLLECT        WHEN
              COLLECT        WHEN
          ... COLLECT <form> WHEN <predicate>)


4.2.2.3. Definite Reference to a Particular Object
4.2.2.3. Definite Reference to a Particular Object
4.2.2.3. Definite Reference to a Particular Object

  In  order  to  simplify  reference  to  a  particular  member of a group,

definite reference may be used.  Such an expression is  written  using  the

      The
      The
word  The followed by the singular form of the group and qualifying phrases

                      For
                      For
(as described for the For statement).  For example, a particular Slot could

be selected by the statement:


       (The Slot with SlotName = NAME)


If there is no object satisfying the specified condition, the value of  the

expression is NIL.


4.2.2.4. WHILE Statement
4.2.2.4. WHILE Statement
4.2.2.4. WHILE Statement

  The format of the WHILE statement is as follows:



        WHILE             DO
        WHILE             DO
       (WHILE <condition> DO <action > ... <action >)
                                    1             n
                                                                         24


The  actions <action > through <action > are executed repeatedly as long as
                    1                 n
                                  DO
                                  DO
<condition> is true.  The keyword DO may be omitted.    The  value  of  the

expression is NIL.


4.2.2.5. REPEAT Statement
4.2.2.5. REPEAT Statement
4.2.2.5. REPEAT Statement

  The format of the REPEAT statement is as follows:



        REPEAT                         UNTIL
        REPEAT                         UNTIL
       (REPEAT <action > ... <action > UNTIL <condition>)
                      1             n



The actions <action > through <action > are repeated (always at least once)
                   1                 n
until  <condition>  is  true.    The  value  of the expression is NIL.  The

        UNTIL
        UNTIL
keyword UNTIL is required.


4.2.3. Messages
4.2.3. Messages
4.2.3. Messages

                     _______
  GLISP supports the Message metaphor, which has its roots in the languages

                                               ______ ________ ___________
SMALLTALK and SIMULA.  These languages provide Object-Centered Programming,

in which objects are thought of as being active entities which  communicate

                          ________
by  sending  each  other  Messages.  The internal structures of objects are

hidden; a program which wishes to access "variables" of an object  does  so

by  sending  messages  to  the  object requesting the access desired.  Each
               11
                            _________
object contains   a list of Selectors, which identify the messages to which

                           _______
the object can respond.  A Message specifies the  destination  object,  the

selector, and any arguments associated with the message.  When a message is

executed  at runtime, the selector is looked up for the destination object;


_______________

  11
    typically by inheritance from some parent in a Class hierarchy
                                                                         25


associated  with  the  selector  is a procedure, which is executed with the

destination object and message arguments as its arguments.



  GLISP  treats  reference  to  properties,  adjectives,   and   predicates

associated  with  an  object  similarly to the way it treats messages.  The

                                                     _________
compiler is able to perform much of the  lookup  of  selectors  at  compile

time,  resulting in efficient code while maintaining the flexibility of the

message metaphor.  Messages can be defined in such a way that they  compile

open,  compile  as  function calls to the function which is associated with

the selector, or compile as messages to be interpreted at runtime.



    _______
  A message in GLISP has the following syntax:


    (SEND                                      )
    (SEND                                      )
    (SEND <object> <selector> <arg > ... <arg >)
                                  1          n

                                       _
                                       _
The keyword "SEND" may be replaced by "_".  The <selector> is assumed to be

quoted.  Zero or more arguments may be specified; the arguments other  than

<selector>  are  evaluated.    <object> is evaluated; if <object> is a non-

atomic expression, it must be enclosed in at least one set of  parantheses,

so that the <selector> will always be the third element of the list.


4.2.3.1. Compilation of Messages
4.2.3.1. Compilation of Messages
4.2.3.1. Compilation of Messages

  When  GLISP encounters a message statement, it looks up the <selector> in

the MSG definition of the <type> of the object  to  which  the  message  is
     12
sent.      Each  <selector>  is paired with the appropriate <response> in a
_______________

  12
    If the  <type>  of  the  destination  object  is  unknown,  or  if  the
<selector>  cannot  be found, GLISP compiles the (SEND ...) statement as if
it is a normal function call.
                                                                         26


                                13
list,  (<selector>  <response>).     Code is compiled depending on the form

of the <response> associated with the <selector>, as follows:


   1. If the <response> is an atom, that atom is taken as the name  of
      a  function  which  is  to be called in response to the message.
      The code which is compiled is a direct call to this function,


          (<response> <object> <arg > ... <arg >)
                                   1          n

   2. If the <response> is a  list,  the  contents  of  the  list  are
      recursively compiled in-line as GLISP code, with the name "self"
      artificially  "bound"  to  the <object> to which the message was
      sent.  Because the compilation is recursive, a  message  may  be
      defined   in   terms   of   other  messages,  substructures,  or
                                                              14
      properties, which may themselves be defined as messages.     The
      outer pair of parentheses of the <response> serves only to bound
      its  contents;  thus,  if the <response> is a function call, the
      function  call  must  be  enclosed  in  an  additional  set   of
      parentheses.



  The  following  examples  illustrate the various ways of defining message

responses.



    (MYMESSAGE    MYRESPONSEFN)

    (SUCCESSOR    (self + 1))

    (MAGNITUDE    ((SQRT X*X + Y*Y)))




_______________

  13
    If an appropriate representation language is provided,  the  <selector>
and  its  associated  <response>  may be inherited from a parent class in a
class hierarchy.

  14
    Such recursive definitions must of course be acyclic.
                                                                         27


In  the first example, a message with <selector> MYMESSAGE is compiled as a

direct call to the function MYRESPONSEFN.    In  the  second  example,  the

SUCCESSOR  message  is  compiled  as  the  sum  of the object receiving the

message (represented by "self") and the constant 1; if the object receiving

the message is bound to the variable J and has a structure of type INTEGER,

the code generated for the SUCCESSOR would be (ADD1 J).  The third  example

illustrates  a call to a function, SQRT, with arguments containing definite

references to X and Y (which presumably are defined as part of  the  object

whose  MAGNITUDE  is  sought).    Note that since MAGNITUDE is defined by a

function call, an extra pair of parentheses is required around the function

call to distinguish it from in-line code.



  The user can determine whether a  message  is  to  be  compiled  open  or

compiled as a function call by the way in which the <response> is specified

in  the  knowledge  base.   Open compilation operates like macro expansion;

since the "macro" is a GLISP expression, it is easy to define messages  and

properties  in  terms of other messages and properties.  The ability to use

definite reference in GLISP makes the definition and use  of  the  "macros"

simple and natural.


4.2.3.2. Compilation of Properties and Adjectives
4.2.3.2. Compilation of Properties and Adjectives
4.2.3.2. Compilation of Properties and Adjectives

  Properties,  Adjectives,  and ISA-adjectives are compiled in the same way

as Messages.  Since the syntax of use of properties and adjectives does not

permit specification of any arguments, the only argument available to  code

or  a  function which implements the <response> for a property or adjective

is the  self  argument, which denotes the object to which the  property  or

adjective  applies.    A <response> which is written directly as GLISP code
                                                                         28


                                  15
will  use the name  self  directly   , as in the SUCCESSOR example above; a

function which is specified as the  <response>  will  be  called  with  the

object as its single argument.


4.2.3.3. Declarations for Message Compilation
4.2.3.3. Declarations for Message Compilation
4.2.3.3. Declarations for Message Compilation

  Declarations   which  affect  compilation  of  Messages,  Adjectives,  or

Properties may be specified following the <response> for a  given  message;

such    declarations    are    in    (INTERLISP)    property-list   format,

<prop ><value > ... <prop ><value >.  The  following  declarations  may  be
     1       1           n       n
specified:


      RESULT
      RESULT
   1. RESULT <type>
                                     ____
      This declaration specifies the type of the result of the message
      or  other  property.    Specification  of result types helps the
      compiler to perform type inference, thus reducing the number  of
      type  declarations needed in user programs.  The RESULT type for
      simple GLISP expressions will be inferred by the  compiler;  the
      RESULT declaration should be used if the <response> is a complex
                                          16
      GLISP expression or a function name.    

      OPEN  T
      OPEN  T
   2. OPEN  T
      This  declaration specifies that the function which is specified
      as the <response> is to be compiled OPEN at each reference.    A
      <response>  which  is  a  list  of GLISP code is always compiled
      OPEN; however, such  a  <response>  can  have  only  the  "self"
      argument.  If it is desired to compile OPEN a Message <response>
      which has arguments besides "self", the <response> must be coded
      as  a  function  (in  order  to bind the arguments) and the OPEN
      declaration must be used.  Functions which are compiled OPEN may
      not be recursive via any chain of OPEN-compiled functions.


_______________

  15
    The name  self  is "declared" by the compiler, and does not have to  be
specified in the Structure Description.

  16
    Alternatively,  the result of a function may be specified by the RESULT
declaration within the function itself.
                                                                         29


4.2.3.4. Operator Overloading
4.2.3.4. Operator Overloading
4.2.3.4. Operator Overloading

  GLISP  provides  operator  overloading for user-defined objects using the

                                                               ________
Message facility.  If an arithmetic operator is defined as the selector  of

a  message  for  a  user  datatype,  an arithmetic subexpression using that

operator will be compiled as if it were a message call with two  arguments.

For  example,  the  type  VECTOR  might  have the declarations and function

definitions below:



       (VECTOR  (CONS (X INTEGER) (Y INTEGER))
          MSG  ((+  VECTORPLUS OPEN T)
                (_+ VECTORINCR OPEN T)) )


       (VECTORPLUS (GLAMBDA (U,V:VECTOR)
           (A VECTOR WITH X = U:X + V:X , Y = U:Y + V:Y) ))

       (VECTORINCR (GLAMBDA (U,V:VECTOR)
           (U:X _+ V:X)
           (U:Y _+ V:Y) ))



With these definitions, an expression involving the operators + or _+  will

be compiled by open compilation of the respective functions.



  The  compound  operators  (_+  +_  _-  -_) are thought of as "destructive

replacement" operators; thus, the expression (U _ U + V) will create a  new

VECTOR  structure  and  assign the new structure to U, while the expression

(U _+ V) will smash the existing structure U, given the definitions  above.

The  convention  of  letting  the  compound  operators specify "destructive

replacement" allows the user to  specify  both  the  destructive  and  non-

destructive  cases.   However, if the compound operators are not overloaded

but the arithmetic operators + and - are overloaded, the compound operators
                                                                         30


are compiled using the definitions of + for _+ and +_, and - for _- and -_.

Thus,  if  only  the  + operator were overloaded for VECTOR, the expression

(U _+ V) would be compiled as if it were (U _ U + V).
                                                                         31


                                 CHAPTER 5
                                 CHAPTER 5
                                 CHAPTER 5

                        CONTEXT RULES AND REFERENCE
                        CONTEXT RULES AND REFERENCE
                        CONTEXT RULES AND REFERENCE

  The ability to use definite reference to features of objects which are in

_______
Context  is  the  key to much of GLISP's power.  At the same time, definite

reference introduces the possibility of ambiguity,  i.e.,  there  might  be

more  than  one object in context which has the specified feature.  In this

chapter, guidelines are presented for use of definite reference which allow

the user to avoid harmful ambiguity.



5.1. Organization of Context
5.1. Organization of Context
5.1. Organization of Context

  The Context maintained by the compiler is organized in  levels,  each  of

which  may  have multiple entries; in most cases, the sequence of levels is

best thought of as a stack.  Searching of the Context proceeds from the top

(nearest) level of the stack to the bottom (farthest) level.    The  bottom

level  of  the  stack  is  composed of the LAMBDA variables of the function

being compiled.  New levels are added  to  the  Context  in  the  following

cases:


   1. When  a  PROG  is compiled.  The PROG variables are added to the
      new level.

             For
             For
   2. When a For loop is compiled.  The "loop index"  variable  (which
      may  be  either a user variable or a compiler variable) is added
      to the new level, so that it is in context during the loop.

             While
             While
   3. When a While loop is compiled.

                              If
                              If
   4. When a new clause of an If statement is compiled.



  When a Message, Property, or  Adjective  is  compiled,  that  compilation

takes  place  in a new context consisting only of the " self " argument and
                                                                         32


other message arguments.



5.2. Rules for Using Definite Reference
5.2. Rules for Using Definite Reference
5.2. Rules for Using Definite Reference

  The   possibility   of   referential  ambiguity  is  disturbing  to  many

programmers; however,  this  problem  is  easily  controlled  in  practice.

First, it should be noted that the traditional methods of unique naming and

complete  path  specification ("PASCAL style") are available, and should be

used whenever there is any possibility of ambiguity.    Second,  there  are

several cases which are guaranteed to be unambiguous:


   1. In compiling GLISP code which implements a Message, Property, or
      Adjective,  only  the " self " argument is in context initially;
      definite reference to any substructure or property of the object
                               17
      is therefore unambiguous.    

               For
               For
   2. Within a For loop, the loop variable is  the  closest  thing  in
      context.

   3. In  many  cases,  a  function will only have a single structured
      argument; in such cases, definite reference is unambiguous.


If "PASCAL" syntax (or  the  equivalent  English-like  form)  is  used  for

references other than the above cases, no ambiguities will occur.










_______________

  17
    Unless  there  are duplicated names in the object definition.  However,
if the same name is used as both a Property and an Adjective, for  example,
it  is  not  considered  a  duplicate  since  Properties and Adjectives are
specified by different source language constructs.
                                                                         33


5.3. Type Inference
5.3. Type Inference
5.3. Type Inference

  In  order  to  interpret  definite references to features of objects, the

                        _____
compiler must know the  types  of the  objects.    However,  explicit  type

specification  can  be  burdensome,  and makes it difficult to change types

without rewriting existing type declarations.  The GLISP compiler  performs

type  inference  in  many  cases, relieving the programmer of the burden of

specifying types explicitly.  The following rules enable the programmer  to

know when types will be inferred by the compiler.


   1. Whenever  a variable is set to a value whose type is known using
      the  _  operator (or one of  its  variants),  the  type  of  the
      variable is inferred to be the type of the value to which it was
      set.

   2. If  a  variable  whose  initial  value  was  NIL  (e.g.,  a PROG
      variable) appears on the left-hand side of  the   _+   operator,
      its  type  is  inferred to be (LISTOF <type>), where  <type>  is
      the type of the right-hand side of the  _+  expression.

   3. Whenever a substructure of a structured object is retrieved, the
      type of the substructure is retrieved also.

   4. Types of infix expressions are inferred.

   5. Types of Properties, Adjectives, and Messages are inferred if:


         a. The  <response>  is GLISP code whose type can be inferred.

         b. The  <response>  has a RESULT declaration associated  with
            it.

         c. The  <response>  is a function whose definition includes a
            RESULT  declaration,  or  whose  property  list contains a
            GLRESULTTYPE declaration.


                                           For
                                           For
   6. The type of the "loop variable" in a For loop is inferred.

            If
            If
   7. If an If statement tests the type of a variable using a " self "
      adjective, the variable is inferred to be of that  type  if  the
      test  is  satisfied.  Similar type inference is performed if the
                                                                 While
                                                                 While
      test of the type of the variable is the  condition  of  a  While
      statement.
                                                                         34


   8. When  possible,  GLISP  infers  the  type  of the function it is
      compiling and adds the type of the result to the  property  list
      of the function name under the indicator GLRESULTTYPE.
                                                                         35


                                 CHAPTER 6
                                 CHAPTER 6
                                 CHAPTER 6

               GLISP AND KNOWLEDGE REPRESENTATION LANGUAGES
               GLISP AND KNOWLEDGE REPRESENTATION LANGUAGES
               GLISP AND KNOWLEDGE REPRESENTATION LANGUAGES

                                   ______  ________
  GLISP   provides  a  convenient  Access  Language  which  allows  uniform

specification of access to objects, without regard to the way in which  the

objects   are   actually  stored;  in  addition,  GLISP  provides  a  basic

______________ ________
Representation Language, in which the structures and properties of  objects

can be declared.  The field of Artificial Intelligence has spawned a number

of  powerful  Representation  Languages,  which provide power in describing

                                                                      _____
large  numbers  of  object  classes  by  allowing  hierarchies   of   Class

descriptions,  in  which  instances  of  Classes can inherit properties and

                                     ______ _________
procedures from parent Classes.  The Access Languages  provided  for  these

Representation  Languages,  however, have typically been rudimentary, often

being no more than variations of LISP's GETPROP and PUTPROP.  In  addition,

by  performing  inheritance of procedures and data values at runtime, these

Representation Languages have often been computationally costly.



  A marriage between GLISP and a Representation  Language  is  particularly

felicitous because the strengths of each can overcome the weaknesses of the

other.      The   Representation   Language,   by  permitting  hierarchical

descriptions of Classes, provides more powerful facilities than  GLISP  for

describing   large   numbers   of   object   Classes.    In  addition,  the

Representation Language can provide the ability  to  interpret  at  runtime

those  messages,  procedures,  and  data values which cannot be resolved at

compile time.   GLISP  provides  a  convenient  and  uniform  language  for

accessing both objects in the Representation Language and LISP objects.  In

addition, GLISP can greatly improve the efficiency of programs which access
                                                                         36


the  representations  by  performing  lookup  of procedures and data in the

                __  _______  ____
Class hierarchy at  compile  time.    Finally,  a  LISP  structure  can  be

           __  ___  ___  __  ____________
specified  as  the  way  of  implementing  instances  of  a  Class  in  the

Representation Language, so that while the objects in such a  class  appear

the  same  as other objects in the Representation Language and are accessed

in the same way, they are actually implemented as LISP  objects  which  are

efficient in both time and storage.


          18
  A  clean    interface  between  GLISP  and  a  Representation Language is

                                             _____
provided.  With  such  an  interface,  each  Class  in  the  Representation

                                     ____
Language  is  acceptable  as a GLISP type.  When the program which is being

compiled specifies an access to an object which is known to be a member  of

some  Class, the interface module for the Representation Language is called

to generate code to perform the access.  The interface module  can  perform

inheritance  within  the  Class  hierarchy,  and  can  call  GLISP compiler

functions to compile code for subexpressions.  Properties, Adjectives,  and

Messages  in  GLISP  format  can  be added to Class definitions, and can be

inherited  by  subclasses  at  compile  time.     In   an   Object-Centered

representation  language  or  other  representation  language  which relies

heavily on procedural inheritance, substantial  improvements  in  execution

speed  can be achieved by performing the inheritance lookup at compile time

and compiling direct procedure  calls  to  inherited  procedures  when  the

procedures  are  static  and  the  type  of  the  object which inherits the


_______________

  18
    Cleanliness is in the eye of the beholder and, being next to Godliness,
                                    __________
difficult to attain.  However, it's relatively clean.
                                                                         37


procedure is known at compile time.



  Specifications  for  an  interface  module  for  GLISP are contained in a
                 19
separate document  .  To date, GLISP has been interfaced to  our  own  GIRL
                                      20
representation language, and to LOOPS.  



































_______________

  19
    to be written.

  20
    LOOPS, a LISP Object Oriented Programming System, is being developed at
Xerox Palo Alto Research Center by Dan Bobrow and Mark Stefik.
                                                                         38


                                 CHAPTER 7
                                 CHAPTER 7
                                 CHAPTER 7

                                GLISP HACKS
                                GLISP HACKS
                                GLISP HACKS

  This chapter discusses some ways of doing things in GLISP which might not

be entirely obvious at first glance.



7.1. Overloading Basic Types
7.1. Overloading Basic Types
7.1. Overloading Basic Types

  GLISP  provides  the ability to define properties of structures described

in the Structure Description language; since the elementary LISP types  are

structures  in  this  language,  objects whose storage representation is an

elementary type can be "overloaded" by specifying properties and  operators

for them.  The following examples illustrate how this can be done.
                                                                         39



    (GLDEFSTRQ


    (ArithmeticOperator  (self ATOM)

       PROP ((Precedence OperatorPrecedenceFn  RESULT INTEGER)
             (PrintForm  ((GETPROP self 'PRINTFORM) or self)) )

       MSG  ((PRIN1      ((PRIN1 the PrintForm)))) )


    (IntegerMod7         (self INTEGER)

       PROP ((Modulus    (7))
             (Inverse    ((If self is ZERO then 0
                                else (Modulus - self))) ))

       ADJ  ((Even       ((ZEROP (LOGAND self 1))))
             (Odd        (NOT Even)))

       ISA  ((Prime      PrimeTestFn))

       MSG  ((+          IMod7Plus  OPEN T  RESULT IntegerMod7)
             (_          IMod7Store OPEN T  RESULT IntegerMod7)) )

    )
    (DEFINEQ

    (IMod7Store  (GLAMBDA (LHS:IntegerMod7 RHS:INTEGER)
             (LHS:self __ (IREMAINDER RHS Modulus)) ))

    (IMod7Plus   (GLAMBDA (X,Y:IntegerMod7)
             (IREMAINDER (X:self + Y:self) X:Modulus) ))
    )


A  few  subtleties of the function IMod7Store are worth noting.  First, the

left-hand-side expression used in storing the  result  is  LHS:self  rather

than  simply  LHS.    LHS  and  LHS:self of course refer to the same actual

                        ____
structure; however, the type of LHS  is  IntegerMod7,  while  the  type  of

LHS:self is INTEGER.  If LHS were used on the left-hand side, since the  _ 

operator  is  overloaded  for IntegerMod7, the function IMod7Store would be

invoked again to perform its own function; since the function  is  compiled

OPEN,  this  would  be  an  infinite  loop.   A second subtlety is that the
                                                                         40


assignment  to  LHS:self must use the self-assignment operator,  __ , since

it is  desired  to  perform  assignment  as  seen  "outside"  the  function

IMod7Store,  i.e.,  in  the  environment  in  which the original assignment

operation was specified.



7.2. Disjunctive Types
7.2. Disjunctive Types
7.2. Disjunctive Types

  LISP programming often involves objects which may in fact be of different

types, but which are for some purposes treated alike.   For  example,  LISP

data  structures  are  typically constructed of CONS cells whose fields may

point to other CONS cells or to ATOMs.   The  GLISP  Structure  Description

language  does  not  permit  the  user to specify that a certain field of a

                         __
                         __
                         __
                         or
                         or
structure is a CONS cell or an ATOM.  However, it is possible to  create  a

GLISP  datatype  which  encompasses  both.    Typically,  this  is  done by

declaring the structure of the object to  be  the  complex  structure,  and

testing  for the simpler structure explicitly.  This is illustrated for the

case of the LISP tree below.



       (LISPTREE  (CONS (CAR LISPTREE) (CDR LISPTREE))

          ADJ    ((EMPTY     (~self)))

          PROP   ((LEFTSON   ((If self is ATOMIC then NIL else CAR)))
                  (RIGHTSON  ((If self is ATOMIC then NIL else CDR)))))




7.3. Generators
7.3. Generators
7.3. Generators

  Often, one would like to define such properties of an object as  the  way

of  enumerating  its  parts in some order.  Such things cannot be specified

directly as properties of the object because they depend  on  the  previous
                                                                         41


state  of  the  enumeration.   However, it is possible to define an object,

associated with the original datatype, which  contains  the  state  of  the

enumeration  and  responds  to  Messages.   This is illustrated below by an

object which searches a tree in Preorder.



    (PreorderSearchRecord  (CONS (Node LISPTREE)
                                 (PreviousNodes (LISTOF LISPTREE)))

       MSG  ((NEXT  ((PROG (TMP)
                        (If TMP_Node:LEFTSON
                            then (If Node:RIGHTSON
                                     then PreviousNodes+_Node)
                                 Node_TMP
                            else TMP-_PreviousNodes
                                 Node_TMP:RIGHTSON) ))))


    (TP (GLAMBDA ((A LISPTREE))
          (PROG (PSR)
             (PSR _ (A PreorderSearchRecord
                       with Node = (the LISPTREE)))
             (While Node (If Node is ATOMIC (PRINT Node))
                         (_ PSR NEXT)) )))



The object class PreorderSearchRecord serves two  purposes:  it  holds  the

state  of  the enumeration, and it responds to messages to step through the

enumeration.  With these  definitions,  it  is  easy  to  write  a  program

involving enumeration of a LISPTREE, as illustrated by the example function

TP  above.    By  being  open-compiled,  messages  to  an  object can be as

efficient as in-line hand coding; yet, the code for the messages  only  has

to be written once, and can easily be changed without changing the programs

which use the messages.
                                                                         42


                                 CHAPTER 8
                                 CHAPTER 8
                                 CHAPTER 8

                             PROGRAM EXAMPLES
                             PROGRAM EXAMPLES
                             PROGRAM EXAMPLES

  In  this  chapter, examples of GLISP object declarations and programs are

presented.  Each example is discussed as a section  of  this  chapter;  the

code  for  the examples and the code produced by the compiler are shown for

each example at the end of the chapter.



8.1. GLTST1 File
8.1. GLTST1 File
8.1. GLTST1 File

  The GLTST1 file illustrates the use of several types of LISP  structures,

and  the use of fairly complex Property definitions for objects.  SENIORITY

of an EMPLOYEE, for example, is defined in terms of the YEAR of DATE-HIRED,

which is  a  substructure  of  EMPLOYEE,  and  the  YEAR  of  the  function
              21
(CURRENTDATE).  



8.2. GLTST2 File
8.2. GLTST2 File
8.2. GLTST2 File

  The  GLTST2  file  illustrates  the  use  of  Messages  for ordinary LISP

objects.  By defining the arithmetic operators as Message selectors for the

object VECTOR, use of vectors in arithmetic expressions  is  enabled;  OPEN

compilation is specified for these messages.



  The  definition  of GRAPHICSOBJECT uses VECTORs as components.  While the

actual structure of a GRAPHICSOBJECT is  simple,  numerous  properties  are


_______________

  21
         ____
    The  type  of  (CURRENTDATE)  must  be known to the compiler, either by
compiling it first, or by including a RESULT declaration  in  the  function
definition  of  CURRENTDATE, or by specifying the GLRESULTTYPE property for
the function name.
                                                                         43


defined for user convenience.  The definition of CENTER is easily stated as

a VECTOR expression.



  The  Messages  of  GRAPHICSOBJECT illustrate how different responses to a

message for different types of objects can be  achieved,  even  though  for

GLISP  compilation  of messages to LISP objects the code for a message must
                            22
be resolved at compile time.      The  DRAW  and  ERASE  messages  get  the

function  to  be  used  from  the  property  list  of the SHAPE name of the

GRAPHICSOBJECT and APPLY it to draw the desired object.



  MOVINGGRAPHICSOBJECT  contains  a   GRAPHICSOBJECT   as   a   TRANSPARENT

component,  so  that  it  inherits  the  properties  of a GRAPHICSOBJECT; a

MOVINGGRAPHICSOBJECT is a GRAPHICSOBJECT which has  a  VELOCITY,  and  will
                                                                         23
move  itself by the amount of its velocity upon the message command STEP.  

The compilation of the message (_ MGO STEP) in the function TESTFN1  is  of

particular  interest.    This  message  is expanded into the sending of the

message  (_ self MOVE VELOCITY)   to   the   MOVINGGRAPHICSOBJECT.      The

MOVINGGRAPHICSOBJECT  cannot  respond  to such a message; however, since it

contains a GRAPHICSOBJECT as a TRANSPARENT  component,  its  GRAPHICSOBJECT




_______________

  22
    For  objects  in  a  Representation  Language, messages may be compiled
directly as LISP  code  or  as  messages  to  be  interpreted  at  runtime,
depending  on  how  much  is known about the object to which the message is
sent and the compilation declarations in effect.

  23
    This example is adapted from the MovingPoint  example  written  by  Dan
Bobrow for LOOPS.
                                                                         44


                         24
responds  to the message.    A GRAPHICSOBJECT responds to a MOVE message by

erasing itself, increasing its START point by the (vector) distance  to  be

moved,  and  then  redrawing  itself.  All of the messages are specified as

being compiled open, so that the short original message actually  generates

a large amount of code.



  A  rectangle  is drawn by the function DRAWRECT.  Note how the use of the

properties defined for a GRAPHICSOBJECT allows an  easy  interface  to  the

system  functions MOVETO and DRAWTO in terms of the properties LEFT, RIGHT,

TOP, and BOTTOM.



























_______________

  24
    TRANSPARENT substructures thus permit procedural  inheritance  by  LISP
objects.
                                                                          i


Table of Contents
Table of Contents
Table of Contents

1. Introduction
1. Introduction
1. Introduction                                                           1

     1.1. Overview of GLISP                                               1
     1.2. Implementation                                                  2

2. Object Descriptions
2. Object Descriptions
2. Object Descriptions                                                    4

     2.1. Declaration of Object Descriptions                              4
     2.2. Structure Descriptions                                          6
          2.2.1. Syntax of Structure Descriptions                         6
          2.2.2. Examples of Structure Descriptions                       8

3. Reference To Objects
3. Reference To Objects
3. Reference To Objects                                                  11

     3.1. Accessing Objects                                              11
     3.2. Creation of Objects                                            13
     3.3. Predicates on Objects                                          13
          3.3.1. Self-Recognition Adjectives                             14

4. GLISP Program Syntax
4. GLISP Program Syntax
4. GLISP Program Syntax                                                  16

     4.1. Function Syntax                                                16
     4.2. Statement Syntax                                               18
          4.2.1. Expressions                                             18
                                                 25
               4.2.1.1. Self-Assignment Operators  
          4.2.2. Compound Statements                                     21
               4.2.2.1. IF Statement                                     21
               4.2.2.2. FOR Statement                                    22
               4.2.2.3. Definite Reference to a Particular Object        23
               4.2.2.4. WHILE Statement                                  23
               4.2.2.5. REPEAT Statement                                 24
          4.2.3. Messages                                                24
               4.2.3.1. Compilation of Messages                          25
               4.2.3.2. Compilation of Properties and Adjectives         27
               4.2.3.3. Declarations for Message Compilation             28
               4.2.3.4. Operator Overloading                             29

5. Context Rules and Reference
5. Context Rules and Reference
5. Context Rules and Reference                                           31

     5.1. Organization of Context                                        31
     5.2. Rules for Using Definite Reference                             32
     5.3. Type Inference                                                 33



_______________

  25
    This section may be skipped by the casual user of GLISP.             20
                                                                         ii


6. GLISP and Knowledge Representation Languages
6. GLISP and Knowledge Representation Languages
6. GLISP and Knowledge Representation Languages                          35

7. GLISP Hacks
7. GLISP Hacks
7. GLISP Hacks                                                           38

     7.1. Overloading Basic Types                                        38
     7.2. Disjunctive Types                                              40
     7.3. Generators                                                     40

8. Program Examples
8. Program Examples
8. Program Examples                                                      42

     8.1. GLTST1 File                                                    42
     8.2. GLTST2 File                                                    42
