Issue FUNCTION-COMPOSITION Writeup

Status: Passed (as amended) Jan 89 X3J13

(And reaffirmed Jun 89 X3J13 that this was the version wanted)


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.