Issue: FUNCTION-COMPOSITION (Version 5)

[masinter.pa@Xerox.COM]

!
Status: Passed (as amended) Jan 89 X3J13
Forum:          Cleanup
Issue:          FUNCTION-COMPOSITION
References:     None
Category:       ADDITION
Edit history:   21-Jun-88, Version 1 by Pitman
                05-Oct-88, Version 2 by Pitman
                 7-Dec-88, Version 3 by Masinter
                12-Dec-88, Version 4 by Masinter (additional comments)
		10-Feb-89, Version 5 (as amended by X3J13 Jan 89)
Related-Issues: TEST-NOT-IF-NOT

Problem Description:

  A number of useful functions on functions are conspicuously
  absent from Common Lisp's basic set. Among them are functions
  which return constant T, constant NIL, and functions which
  combine functions in common, interesting ways.

Proposal (FUNCTION-COMPOSITION:JAN89-X3J13):

  Add the following functions:

   COMPLEMENT function					[Function]

    Returns a function whose value is the same as the NOT of the
    given function applied to the same arguments.

   CONSTANTLY value						[Function]

    Returns a function whose value is always VALUE.

Examples:

  (MAPCAR #'(LAMBDA (X) (DECLARE (IGNORE X)) T) '(3 A 4.3))
  ==
  (MAPCAR (CONSTANTLY T) '(3 A 4.3))
  => (T T T)

  (FIND-IF-NOT #'ZEROP '(0 0 3))
  ==
  (FIND-IF (COMPLEMENT #'ZEROP) '(0 0 3))
  => 3

Rationale:

  The presence of these functions will contribute to syntactic
  conciseness in some cases. 

Current Practice:

  No Common Lisp implementations provide these functions,
  but they do exist in the T language.

Cost to Implementors:

  A straightforward implementation is simple to cook up. The definitions
  given here would suffice. Typically some additional work might be
  desirable to make these open code in interesting ways.

  (DEFUN COMPLEMENT (FUNCTION)
    #'(LAMBDA (&REST ARGUMENTS)
        (NOT (APPLY FUNCTION ARGUMENTS))))

  (DEFUN CONSTANTLY (VALUE)
    #'(LAMBDA (&REST ARGUMENTS) 
        (DECLARE (IGNORE ARGUMENTS))
        VALUE))

Cost to Users:

  None. This change is upward compatible.

Cost of Non-Adoption:

  (COMPLEMENT BENEFITS)

Benefits:

  Some code would be more clear. 
  Some compilers might be able to produce better code.

  Takes a step toward being able to flush the -IF-NOT functions
  and the :TEST-NOT keywords, both of which are high on the list
  of what people are referring to when they say Common Lisp is
  bloated by too much garbage.

Aesthetics:

  In situations where these could be used straightforwardly, the
  alternatives are far less perspicuous.

Discussion:

  Several additional functions (COMPOSE, CONJOIN) were
 considered and rejected at the Jan 89 X3J13 meeting.