## 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
Edit history:   21-Jun-88, Version 1 by Pitman
05-Oct-88, Version 2 by Pitman
7-Dec-88, Version 3 by Masinter
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):

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.

(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.
```