Issue FUNCTION-TYPE Writeup

Issue:        FUNCTION-TYPE

References:   functions (p32), types (p33), FUNCTIONP (p76),

              SYMBOL-FUNCTION (p90), APPLY (p107), COERCE (pp51-52)

Category:     CHANGE

Edit History: 26-Feb-87, Version 1 by Gabriel

              15-Mar-87, Version 2 by Cleanup Committee

              10-May-87, Version 3 by Fahlman

              29-May-87, Version 4 by Masinter (incorporate comments)

              15-Jun-87, Version 5 by Fahlman (include two options)

              23-Oct-87, Version 6 by Masinter (only STRICT-REDEFINITION)

              09-Nov-87, Version 7 by Masinter (minor cleanup)

              14-Nov-87, Version 8 by Pitman (major restructuring)

              13-Feb-88, Version 9 by Masinter, (add back 2nd option)

              19-May-88, Version 10 by Masinter, (modify as per X3J13)

              24-May-88, Version 11 by van Roggen

                            (don't coerce lists, relax SYMBOL-FUNCTION reqs)

		   4-Sep-88, Version 12 by Masinter

		 	(incorporate amendments adopted at June 88 X3J13)


Problem Description:


 The definition of the term ``function'' in CLtL includes all symbols and

 many lists in addition to `true' functions.


 Also, page 47 of CLtL states that the FUNCTION type specifier can only

 be used for declaration and not for discrimination. Some of the original

 Common Lisp designers maintain that this restriction on the use of the

 FUNCTION specifier was meant to apply only to long-form FUNCTION

 specifiers, but since this intent was not explicitly stated, the status

 of FUNCTION as a type is blurred. 


 A consequence of the p47 confusion is that (FUNCTIONP x) cannot portably

 be relied upon to be equivalent to (TYPEP x 'FUNCTION).


Proposal FUNCTION-TYPE:X3J13-MARCH-88


This proposal is basically the STRICT-REDEFINITION proposal of version 9

of this issue, correcting a few typos, changing section 2E as

agreed upon at X3J13 March 1988, allowing symbols but not lists to

be FUNCALLed or APPLYed, and relaxing some SYMBOL-FUNCTION/FBOUNDP

requirements.


 1.  Redefine the type FUNCTION so that it can be used for discrimination

     as well as declaration.


    1a. The types CONS, SYMBOL, ARRAY, NUMBER, CHARACTER, and FUNCTION

        are pairwise disjoint.  In particular, a list may not be used

        to implement any FUNCTION subtype.


    1b. Define that the type COMPILED-FUNCTION is a subtype of FUNCTION.

        Implementations are free to define other subtypes of FUNCTION.


 2. Define that a ``function'' as used throughout the CLtL is restricted

    to be exactly those objects of type FUNCTION.


    2a. This type no longer includes objects of type SYMBOL or lists

        whose CAR is LAMBDA.


    2b. The behavior of FUNCTIONP is defined to be exactly equivalent to

        #'(LAMBDA (X) (TYPEP X 'FUNCTION)).  This is an incompatible

        change.


    2c. Clarify that the list form of the FUNCTION type specifier may

        still only be used for declaration.


    2d. Clarify that the symbol form of the FUNCTION type specifier may

        be used for type discrimination.


    2e. FUNCALL and APPLY and all Common Lisp functions that

	take function arguments to also take a symbol, which will

	be coerced to a function as if by SYMBOL-FUNCTION.


    2f. This is an incompatible change in that it is an error to pass

	  anything other than a function or symbol as the functional

	  argument.


 3. Clarify that the result of a FUNCTION special form must be a function.


    3a. This implies that some (FUNCTION name) may be implicitly interpreted

	as (THE FUNCTION (FUNCTION name)). 


 4. Clarify that it is an error to use the special form FUNCTION on a

    symbol that does not denote a function in the lexical environment in

    which the special form appears. Specifically, it is an error to use the

    FUNCTION special form on a symbol that denotes a macro or special form.

    

    4a. Some implementations may choose not to signal this error for

        performance reasons, but implementations are forbidden from

        defining the failure to signal an error as a `useful' behavior.


 5. Clarify that FBOUNDP must return true for a symbol naming a macro or

    a special form, and that it is permissible to call SYMBOL-FUNCTION

    on any symbol for which FBOUNDP returns true.


    5a. The value returned by SYMBOL-FUNCTION when FBOUNDP returns true

        but the symbol denotes a macro or special form is not well-defined,

        but SYMBOL-FUNCTION will not signal an error. 


    5b. SETF of SYMBOL-FUNCTION requires a FUNCTION as the new value.

	It is an error to set the SYMBOL-FUNCTION of a symbol to a

	symbol or a list or the value returned by SYMBOL-FUNCTION on

	the name of a macro or a special form.


    5c. The motivation for this distinction between FUNCTION and 

	SYMBOL-FUNCTION is that FUNCTION is intended for day-to-day

	use within programs while SYMBOL-FUNCTION is a data structure

	accessor used primarily for meta-level applications and not

	recommended for general use. It is provided primarily to

	complete the set of accessors on symbols.


 6. COERCE is extended to allow objects to be coerced to type FUNCTION.


    6a. (COERCE symbol 'FUNCTION) extracts the SYMBOL-FUNCTION of the

        given symbol, signalling an error if the symbol is not FBOUNDP or

	if the symbol names a macro or a special-form.


    6b. (COERCE x 'FUNCTION), where the value of x is a list that

		 begins with LAMBDA, will return a FUNCTION similar to

		 (EVAL '(FUNCTION ,x)).


 7. Clarify that the value of *MACROEXPAND-HOOK* is first coerced to a

    function before being called as the expansion interface hook by

    MACROEXPAND-1.


Rationale:


 The fuzzy definition of ``function'' has descended from older dialects of

 Lisp, such as Maclisp. Many places in existing code make assumptions about

 the current meaning, making any change painful.


 It is very important both for documentation clarity and for program type

 discrimination (such as CLOS) to have a clear term which denotes a 

 ``true function.''


 This proposal is a compromise between a CONSERVATIVE proposal (which left

 FUNCTION alone and introduced a new type), and a STRICT-REDEFINITION proposal,

 which incompatibly changed not only the FUNCTION type and SYMBOL-FUNCTION,

 but also the behavior of FUNCALL, APPLY and functions with functional

 arguments.


 For compatibility reasons symbols are still acceptable to FUNCALL et al.,

 but for aesthetic reasons lambda-expressions (lists whose CAR is LAMBDA

 and whose CADR is a list) are no longer acceptable.


Current Practice:


 In some implementations, (TYPEP x 'FUNCTION) signals an error.

 In some implementations, (TYPEP x 'FUNCTION) is true for values

   returned by FUNCTION, symbols that are FBOUNDP, and lambda expressions. 

 In some implementations, (TYPEP x 'FUNCTION) is true only for values

   returned by FUNCTION.


 Implementations vary on what my go into the function cell, depending on

 how much error checking they want to have to do at function call time, and

 depending on whether they store other kinds of information (such as special

 form information) in the function cell.


 Few current Common Lisp implementations have exactly the

 semantics described in this proposal.


Cost to Implementors:


 Bringing type predicates (FUNCTIONP, etc.) and higher order functions

 (APPLY, etc.) into compliance should require little effort in most

 implementations.


 Compiled functions are true functions in almost all current

 implementations, but in many implementations, interpreted functions and

 closures stored in the function cell of a symbol are represented as lists.

 Under this proposal, this representation would have to be different

 (implemented either as structures or as some special internal data type).

 The behavior of COMPILE, STEP, TRACE, and possibly ED would have to be 

 modified to deal with functions that are not lists (but from which the

 list form can be reconstructed if necessary).


Cost to Users:


 The changes to FUNCTIONP and the FUNCTION type declaration are relatively easy

 to deal with. 


 Because CLtL's language was somewhat fuzzy about what might go into the

 function cell of a symbol, some code that explicitly deposited symbols

 or lists in a symbol's function cell, or expected lists back, will

 have to change. Such code was already not portable, however, since some

 implementations signal an error when this is done.


 The original STRICT-REDEFINITION proposal required users to deal with

 the use of symbols and lambda-expressions as functional arguments.  However

 this proposal is compatible with current CLtL definition in the use of

 symbols, which would be the hardest change to make.  There are probably

 relatively few uses of lambda-expressions as ``functions'', which can

 be dealt with by (EVAL `(FUNCTION ,lambda-expresssion)).


Benefits:


 The term ``function'' would be given a useful and precise meaning.

 The FUNCTION datatype would be useful for type discrimination in CLOS.


 The type hierarchy would be simplified.


 This proposal brings Common Lisp slightly closer to Scheme and the

 work of the EuLisp committee. Scheme, for example, also has the concept

 of a ``procedure'' which is compatible with the FUNCTION type.



Aesthetics:


 This proposal improves the aesthetics of the language.


 Lambda-expressions do not obey the normal, apparent scoping rules because

 free variables cannot refer to lexical bindings.  This is because

 coercing a list to a function would mean (EVAL `(FUNCTION ,list)).


 The following code does -not- count the number of nodes in a graph:


  (LET ((COUNTER 0))

    (TRAVERSE-THING '(LAMBDA (NODE) (INCF COUNTER))

                    (THING-ROOT)))


 since it is not the same as


  (LET ((COUNTER 0))

    (TRAVERSE-THING #'(LAMBDA (NODE) (INCF COUNTER))

                    (THING-ROOT)))


 which does pass around a closure incrementing the LET variable.

 (These examples assume COUNTER wasn't PROCLAIMed SPECIAL.)


 Making the coercion of lambda-expressions to functions explicit with

 the use of EVAL will encourage less confusing code and also highlight

 that use of EVAL.



Discussion:


This issue has been discussed at great length; this section attempts

only to summarize the important points.


There is general agreement that the definition of the FUNCTION data type

must be clarified or revised. The cleanup of the type hierarchy is important

to the CLOS group.


The description of COMPILE must be changed, since it is no longer

meaningful to speak of a symbol with a definition that "is a

lambda-expression".  We believe this is a subject for a separate

proposal, as the behavior of COMPILE needs additional clarification.


Many different alternatives have been discussed both in the cleanup committee

and X3J13. Two proposals were circulated at the March 1988 meeting of X3J13;

this version is the result of discussions at that meeting. It is a compromise

between the conflicting goals of backward compatibility, flexibility in the

language, and simple semantics.

 

This proposal does not address the issue of when coercion to functions occur.

For example, it is allowed to write


(MAPCAR 'FROB my-list)


It is not specified when the coercion of FROB to its SYMBOL-FUNCTION 

occurs. For example, 


(DEFUN FROB (X) 

   (WHEN (> X 0) (SETF (SYMBOL-FUNCTION 'FROB) #'(LAMBDA (X) NIL)))

   T)


(MAPCAR 'FROB '(-1 -1 1 1))


may return different results if MAPCAR coerces its functional argument

once rather than for each element. This may require a separate

cleanup issue.