17 Symbolic Mode

At the system level, REDUCE is based on a version of the programming language Lisp known as Standard Lisp which is described in J. Marti, Hearn, A. C., Griss, M. L. and Griss, C., “Standard LISP Report" SIGPLAN Notices, ACM, New York, 14, No 10 (1979) 48-68. We shall assume in this section that the reader is familiar with the material in that paper. This also assumes implicitly that the reader has a reasonable knowledge about Lisp in general, say at the level of the LISP 1.5 Programmer’s Manual (McCarthy, J., Abrahams, P. W., Edwards, D. J., Hart, T. P. and Levin, M. I., “LISP 1.5 Programmer’s Manual”, M.I.T. Press, 1965) or any of the books mentioned at the end of this section. Persons unfamiliar with this material will have some difficulty understanding this section.

Although REDUCE is designed primarily for algebraic calculations, its source language is general enough to allow for a full range of Lisp-like symbolic calculations. To achieve this generality, however, it is necessary to provide the user with two modes of evaluation, namely an algebraic mode and a symbolic mode. To enter symbolic mode, the user types symbolic; (or lisp;) and to return to algebraic mode one types algebraic;. Evaluations proceed differently in each mode so the user is advised to check what mode he is in if a puzzling error arises. He can find his mode by typing

        eval_mode;

The current mode will then be printed as ALGEBRAIC or SYMBOLIC.

Expression evaluation may proceed in either mode at any level of a calculation, provided the results are passed from mode to mode in a compatible manner. One simply prefixes the relevant expression by the appropriate mode. If the mode name prefixes an expression at the top level, it will then be handled as if the global system mode had been changed for the scope of that particular calculation.

For example, if the current mode is algebraic, then the commands

        symbolic car ’(a);  
        x+y;

will cause the first expression to be evaluated and printed in symbolic mode and the second in algebraic mode. Only the second evaluation will thus affect the expression workspace. On the other hand, the statement

        x + symbolic car ’(12);

will result in the algebraic value x+12.

The use of symbolic (and equivalently algebraic) in this manner is the same as any operator. That means that parentheses could be omitted in the above examples since the meaning is obvious. In other cases, parentheses must be used, as in

        symbolic(x := ’a);

Omitting the parentheses, as in

        symbolic x := a;

would be wrong, since it would parse as

        symbolic(x) := a;

For convenience, it is assumed that any operator whose first argument is quoted is being evaluated in symbolic mode, regardless of the mode in effect at that time. Thus, the first example above could be equally well written:

        car ’(a);

Except where explicit limitations have been made, most REDUCE algebraic constructions carry over into symbolic mode. However, there are some differences. First, expression evaluation now becomes Lisp evaluation. Secondly, assignment statements are handled differently, as we shall discuss shortly. Thirdly, local variables and array elements are initialized to nil rather than 0. (In fact, any variables not explicitly declared INTEGER are also initialized to nil in algebraic mode, but the algebraic evaluator recognizes nil as 0.) Finally, function definitions follow the conventions of Standard Lisp.

To begin with, we mention a few extensions to our basic syntax which are designed primarily if not exclusively for symbolic mode.

• 17 Symbolic Mode
• 17.1 Symbolic Infix Operators
• 17.2 Symbolic Expressions
• 17.3 Quoted Expressions
• 17.4 Lambda Expressions
• 17.5 Symbolic Assignment Statements
• 17.6 FOR EACH Statement
• 17.7 Symbolic Procedures
• 17.8 Standard Lisp Equivalent of Reduce Input
• 17.9 Communicating with Algebraic Mode
• 17.10 Rlisp ’88
• 17.11 References

17.1 Symbolic Infix Operators

There are three binary infix operators in REDUCE intended for use in symbolic mode, namely . (cons), eq and memq. The precedence of these operators was given in another section.

17.2 Symbolic Expressions

These consist of scalar variables and operators and follow the normal rules of the Lisp meta language.

Examples:

        x  
        car u . reverse v  
        simp (u+v^2)

17.3 Quoted Expressions

Because symbolic evaluation requires that each variable or expression has a value, it is necessary to add to REDUCE the concept of a quoted expression by analogy with the Lisp quote function. This is provided by the single quote mark ’. For example,

’a    		represents the Lisp S-expression	(quote a)
’(a b c)   	represents the Lisp S-expression	(quote (a b c))

Note, however, that strings are constants and therefore evaluate to themselves in symbolic mode. Thus, to print the string ~A String~, one would write

        prin2 ~A String~;

Within a quoted expression, identifier syntax rules are those of REDUCE. Thus (A !. B) is the list consisting of the three elements A, ., and B, whereas (A . B) is the dotted pair of A and B.

17.4 Lambda Expressions

lambda expressions provide the means for constructing Lisp lambda expressions in symbolic mode. They may not be used in algebraic mode.

Syntax:

⟨LAMBDAexpression⟩ ::=  LAMBDA ⟨varlist⟩⟨terminator⟩⟨statement⟩

where

⟨varlist⟩ ::= (⟨variable⟩,…,⟨variable⟩)

e.g.,

        lambda (x,y); car x . cdr y;

is equivalent to the Lisp lambda expression

        (lambda (x y) (cons (car x) (cdr y)))

The parentheses may be omitted in specifying the variable list if desired.

lambda expressions may be used in symbolic mode in place of prefix operators, or as an argument of the reserved word function.

In those cases where a lambda expression is used to introduce local variables to avoid recomputation, a where statement can also be used. For example, the expression

        (lambda (x,y); list(car x,cdr x,car y,cdr y))  
            (reverse u,reverse v)

can also be written

      {car x,cdr x,car y,cdr y} where x=reverse u,y=reverse v

Where possible, where syntax is preferred to lambda syntax, since it is more natural.

17.5 Symbolic Assignment Statements

In symbolic mode, if the left side of an assignment statement is a variable, a setq of the right-hand side to that variable occurs. If the left-hand side is an expression, it must be of the form of an array element, otherwise an error will result. For example, x:=y translates into (SETQ X Y) whereas a(3) := 3 will be valid if a has been previously declared a single dimensioned array of at least four elements.

17.6 FOR EACH Statement

The for each form of the for statement, designed for iteration down a list, is more general in symbolic mode. Its syntax is:

        FOR EACH ID:identifier {IN|ON} LST:list  
            {DO|COLLECT|JOIN|PRODUCT|SUM} EXPRN:S-expr

As in algebraic mode, if the keyword in is used, iteration is on each element of the list. With on, iteration is on the whole list remaining at each point in the iteration. As a result, we have the following equivalence between each form of for each and the various mapping functions in Lisp:

	DO	COLLECT	JOIN
IN	MAPC	MAPCAR	MAPCAN
ON	MAP	MAPLIST	MAPCON

Example: To list each element of the list (a b c):

        for each x in ’(a b c) collect list x;

17.7 Symbolic Procedures

All the functions described in the Standard Lisp Report are available to users in symbolic mode. Additional functions may also be defined as symbolic procedures. For example, to define the Lisp function ASSOC, the following could be used:

        symbolic procedure assoc(u,v);  
           if null v then nil  
            else if u = caar v then car v  
            else assoc(u, cdr v);

If the default mode were symbolic, then symbolic could be omitted in the above definition. macros may be defined by prefixing the keyword procedure by the word macro. (In fact, ordinary functions may be defined with the keyword expr prefixing procedure as was used in the Standard Lisp Report.) For example, we could define a macro conscons by

        symbolic macro procedure conscons l;  
           expand(cdr l,’cons);

Another form of macro, the smacro is also available. These are described in the Standard Lisp Report. The Report also defines a function type fexpr. However, its use is discouraged since it is hard to implement efficiently, and most uses can be replaced by macros. At the present time, there are no fexprs in the core REDUCE system.

17.8 Standard Lisp Equivalent of Reduce Input

A user can obtain the Standard Lisp equivalent of his REDUCE input by turning on the switch defn (for definition). The system then prints the Lisp translation of his input but does not evaluate it. Normal operation is resumed when defn is turned off.

17.9 Communicating with Algebraic Mode

Not initially supported by Reduce.jl parser, see upstream docs for more information.

17.10 Rlisp ’88

Rlisp ’88 is a superset of the Rlisp that has been traditionally used for the support of REDUCE. It is fully documented in the book Marti, J.B., “RLISP ’88: An Evolutionary Approach to Program Design and Reuse”, World Scientific, Singapore (1993). Rlisp ’88 adds to the traditional Rlisp the following facilities:

1. more general versions of the looping constructs for, repeat and while;
2. support for a backquote construct;
3. support for active comments;
4. support for vectors of the form name[index];
5. support for simple structures;
6. support for records.

In addition, “-” is a letter in Rlisp ’88. In other words, A-B is an identifier, not the difference of the identifiers A and B. If the latter construct is required, it is necessary to put spaces around the - character. For compatibility between the two versions of Rlisp, we recommend this convention be used in all symbolic mode programs.

To use Rlisp ’88, type on rlisp88;. This switches to symbolic mode with the Rlisp ’88 syntax and extensions. While in this environment, it is impossible to switch to algebraic mode, or prefix expressions by “algebraic”. However, symbolic mode programs written in Rlisp ’88 may be run in algebraic mode provided the rlisp88 package has been loaded. We also expect that many of the extensions defined in Rlisp ’88 will migrate to the basic Rlisp over time. To return to traditional Rlisp or to switch to algebraic mode, say “off rlisp88;”.

17.11 References

There are a number of useful books which can give you further information about LISP. Here is a selection:

Allen, J.R., “The Anatomy of LISP”, McGraw Hill, New York, 1978.

McCarthy J., P.W. Abrahams, J. Edwards, T.P. Hart and M.I. Levin, “LISP 1.5 Programmer’s Manual”, M.I.T. Press, 1965.

Touretzky, D.S, “LISP: A Gentle Introduction to Symbolic Computation”, Harper & Row, New York, 1984.

Winston, P.H. and Horn, B.K.P., “LISP”, Addison-Wesley, 1981.