FUNCTION-TYPE
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.
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.
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.
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).
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)).
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.
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.
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.