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                      Learn LOGO Before LISP
                         by Mike McMillan
                  March 1987 issue of AI EXPERT

                      Sidebars 1, 2, and 3


Sidebar 1

The following is a list of the LISP primitives,  predicates,
functions and their Logo equivalents. Those functions that do not
have  a  Logo  equivalent  have been noted.  Please remember that
most,  if not all,  of the LISP functions that are not duplicated
in Logo can be written in Logo. Logo is powerful enough to create
functions that behave like their LISP equivalents.


The  LISP  function  is  listed first,  followed by the Logo
equivalent or N/A, meaning Logo does not have that function.

          CAR - FIRST                        GET - GETPROP

          CDR - BUTFIRST                     PUTPROP - PUTPROP

          CXXXR - FIRST BUTFIRST...BUTFIRST  REMPROP - REMPROP

          LAST - LAST                        PLIST - PLIST

          CONS - FPUT (FIRST PUT)            ATOM - WORDP

          APPEND - LPUT (LAST PUT)           NULL - EMPTYP

          LIST - LIST                        NUMBERP - NUMBERP

          NCONC - N/A (NCONC IN EXPERLOGO)   SYMBOLP - N/A

          RPLACA - N/A (RPLACA IN EXPERLOGO) LISTP - LISTP

          RPLACD - N/A (RPLACD IN EXPERLOGO) EQUAL - EQUALP

          NREVERSE - N/A                     MEMBER - MEMBERP

          ASSOC - N/A (FINDKEY IN EXPERLOGO) GREATERP - GREATERP

          LENGTH - COUNT                     LESSP - LESSP

          REVERSE - N/A(REVERSE IN EXPERLOGO)ZEROP - N/A

          SUBST - N/A (SUBST IN EXPERLOGO)   MINUSP - N/A

          LCONC - N/A                        PLUSP - N/A

          TCONC - N/A                        EVENP - N/A

          SETQ - MAKE                        ODDP - N/A

          SETF - N/A                         COND - IF

          SET - N/A                          IF - IF

          DEFUN - TO

This is not an exhaustive list; several of the more  compli-cated
or lesser-used LISP  functions  have  been  left  out.  The
reader should see from this list,  though,  just how much of LISP
has been implemented in Logo.



Sidebar 2
Writing LISP Functions in Logo

Many of the LISP primitives  and  predicates  that  are  not
duplicated  in  Logo  can  be written in Logo.  Logo's power as a
symbolic computation language allows the programmer to do this.

For example,  in LISP there is a powerful  primitive  called
ASSOC. ASSOC looks for a key that is part of an association list.
For example, if we set up the following association:

                    (SETQ 'JANE '((OCCUPATION STUDENT)
                                 (MAJOR COMPUTER-SCIENCE)
                                 (LANGUAGE LISP)))


ASSOC can retrieve the information we have set up.

Writing in LISP: (ASSOC 'LANGUAGE SALLY),  the computer will
respond: (LANGUAGE LISP).

ASSOC is not defined in Logo. It is,  however,  very easy to
create.  We  set  up  the  association  by  writing:  MAKE "SALLY
[[OCCUPATION STUDENT] [MAJOR COMPUTER.SCIENCE] [LANGUAGE  LISP]].
We define the ASSOC primitive as follows:

                    TO ASSOC :KEY :OBJECT
                    IF EMPTYP :OBJECT [OUTPUT "? STOP]
                    IF  EQUALP  FIRST  FIRST  :OBJECT  :KEY  [OUTPUT  FIRST
          :OBJECT STOP]
                    OUTPUT ASSOC :KEY BF :OBJECT
                    END

Our Logo ASSOC goes through the  association  list    recur-
sively,  checking  each  key  to  see  if it matches the argument
given.  If there is nothing in the association  list,  or  if  it
cannot  find a match,  ASSOC will output a question mark.  When a
match is found, the key and the object are output and the  proce-
dure stops.

ASSOC is a fairly complicated example.  Several of the  LISP
primitives  that  do  not have Logo equivalents can be defined in
one or two lines of Logo code.  For example,  to define the  Logo
version of LISP's ZEROP, we write:

                    TO MY-ZEROP: NUMBER
                    IF EQUALP :NUMBER "0 [OUTPUT "TRUE]
                    IF  EQUALP  LAST  :NUMBER  "0  [OUTPUT  "TRUE]  [OUTPUT
          "FALSE]
                    END

The extensibility of Logo is helpful in the study  of  LISP. the
student  trying to model LISP expressions in Logo will often need
to write LISP primitives and predicates in Logo in order  to use
them. I found in my own study that writing a LISP function in
Logo gave me a deeper understanding of that function.  It takes a
higher magnitude of understanding to write  a  function  than  it
does just to use the function.



Sidebar 3
ExperLogo - A LISP-like Logo

One  version of Logo stands out from the others as being the most
LISP-like in its form and structure - ExperLogo from  Exper-
Telligence.  ExperLogo's LISPness is apparent in  several    dif-
ferent  areas - from its rules of evaluation to its toolchest  of
primitives and predicates.

The first noticeable LISP-like feature of ExperLogo  is  the way
it evaluates its expressions.  In ExperLogo every expression is
evaluated,  meaning that primitives and predicates will  print
the  results  of  their evaluation on the screen without the need
for a PRINT command.  This element of first-classness is an   im-
portant  feature  of  LISP  that is not found in most versions of
Logo.

For example,  in most Logos to print the value of a variable the
variable  has  to be combined with a PRINT command,  such as
PRINT :PEOPLE.  In ExperLogo only the variable has to be written,
as  in  :PEOPLE,  which  is much like the way it would be done in
LISP - (PEOPLE).

ExperLogo is also more LISP-like in the way values are bound to
variables.  In most Logos a value is bound to  a  variable  by
writing  MAKE  "NAME "TOM.  In ExperLogo you can write: MAKE NAME
TOM. In LISP it would be (SETQ NAME TOM).

Expressions that evaluate to  true  or  false  in  ExperLogo
return values in the same manner as LISP,  either T or NIL, which
is unlike most Logos, which return TRUE or FALSE.

Another area of ExperLogo that makes  it  so  comparable  to LISP
is its expanded list of LISP-like primitives and predicates. In
most  versions  of  Logo  there is not a primitive similar to
LISP's REVERSE,  which takes the elements of a list  and  reverse
their order. ExperLogo has REVERSE.

In LISP the primitive SUBST is used to substitute a new  ex-
pression  for an old expression in some target expression,  say a
list.  Most versions of Logo do not have this powerful primitive;
ExperLogo does.

A  very  powerful  primitive  in LISP is ASSOC (discussed in
another sidebar).  Most Logos do not have this primitive,  though
it  can  be  written in Logo.  ExperLogo has this primitive under
another name - FINDKEY. FINDKEY works just like ASSOC; it takes a
key word (atom) and an expression as arguments and then finds the
element that goes with the key.

Here is a list of functions in ExperLogo which are not found in
other Logos but can be found in LISP: ELEMS,  FINDKEY,  NCONC,
NPUT,  PAIRLIST,  REMOVE,  REVERSE, RPLACA, RPLACD, SORT, SORTKEY
and SUBST.  Some of these functions do not duplicate  their  LISP
equivalent's  name,  but in following the Logo tradition are more
mnemonic, hence easier to remember and understand.

ExperLogo at this time is available only for the  Macintosh, but
if  you  have  a  Mac and are looking for a Logo with a high
degree of LISP similarity,  I would highly recommend  looking  at
this excellent Logo implementation.