predicates

Documentation

Deprecated - For basic logic combinators, use conjoin, disjoin, and negate from racket/function instead. For predicate-constructing variants of eq? and friends, use a simple lambda shorthand library like fancy-app instead. For the more complex combinators, consider them unmaintained.

A racket package for creating predicates in a point-free style.

A predicate is a function that takes one argument and returns either true or false. You can think of it as a question, such as even? which returns true for even numbers and false for odd numbers. This package provides several tools for working with predicates.

Predicates can be combined logically

(filter (or? symbol? string?) '(1 a 3 "blah" b 4 5))
; -> '(a "blah" b)

(filter (and? number? even?) '(a b 2 5 6 "foo" bar 9 8))
; -> '(2 6 8)

(filter (not? symbol?) '(a b 2 3 "baz" d))
; -> '(2 3 "baz")

Predicates can be created to compare things

(filter (<? 5) '(3 4 5 6 7))
; -> '(3 4)
(filter (eq?? 'foo) '(foo bar baz))
; -> '(foo)
(define digit? (and? exact-positive-integer? (<? 10)))
(filter digit? '(a b 2 5 c "foo" 12 15 8))
; -> '(2 5 8)

Predicates can be created and combined to query lists

(nonempty-list? '(a b c)) ; -> #t
(nonempty-list? '()) ; -> #f
(nonsingular-list? '(a b c)) ; -> #t
(nonsingular-list? '(a)) ; -> #f

(define second-number? (second? number?))
(second-number? '(1 2 3)) ; -> #t
(second-number? '(1 a 3)) ; -> #f

(define three-nums? (listof? number? number? number?))
(three-nums? '(1 2 3)) ; -> #t
(three-nums? '(1 2 3 4)) ; -> #f
(three-nums? '(a b 2)) ; -> #f

(define starts-with-sym-str? (list-with-head? symbol? string?))
(starts-with-sym-str? '(a "foo" 1 2 3)) ; -> #t
(starts-with-sym-str? '(a 1 2 3)) ; -> #f

((length>? 2) '(a b c)) ; -> #t
((length>? 2) '(a)) ; -> #f

Once you've got all these fancy predicates, you can use them to add conditional logic to functions.

(define (halve x) (/ x 2))
(define (triple-add1 x) (add1 (* x 3)))
(define collatz (if? even? halve triple-add1))
(collatz 4) ; -> 2
(collatz 5) ; -> 16
(collatz 7) ; -> 22
(collatz 1) ; -> 4

Or use them as loop conditions in higher-order loop construction functions.

(define last (compose first (while? nonsingular-list? rest)))
(last '(1 2 3 4 5))
(define (last? p) (compose p last))
((last? even?) '(1 2 3 4 5))   ; -> #f
((last? even?) '(1 2 3 4 5 6)) ; -> #t

To install, run raco pkg install predicates. Then to use in a module, (require predicates).