git @ Cat's Eye Technologies Robin / master stdlib / prefix-p.robin
master

Tree @master (Download .tar.gz)

prefix-p.robin @master — raw · history · blame 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
;'<<SPEC'

<!--
Copyright (c) 2012-2024, Chris Pressey, Cat's Eye Technologies.
This file is distributed under a 2-clause BSD license.  See LICENSES/ dir.
SPDX-License-Identifier: LicenseRef-BSD-2-Clause-X-Robin
-->

### `prefix?` ###

    -> Tests for functionality "Evaluate Robin Expression (with List)"

`prefix?` evaluates its first and second arguments to obtain lists.
It then evaluates to `#t` if the first list is a prefix of the second
list, `#f` otherwise.  A list A is a prefix of a list B if A is `empty?`,
or the head of A is `equal?` to the head of B and the tail of A is a
prefix of the tail of B.

    | (prefix? (list 1 2 3) (list 1 2 3 4 5 6))
    = #t

    | (prefix? (list 1 2 5) (list 1 2 3 4 5 6))
    = #f

    | (prefix? () (list 1 2 3 4 5 6))
    = #t

    | (prefix? () (literal schpritz))
    = #t

    | (prefix? (list 1 2 3) (list 1 2 3))
    = #t

    | (prefix? (list 1 2 3 4) (list 1 2 3))
    = #f

'<<SPEC'

(define prefix? (fun (la lb)
  (bind prefix?-r (fun (self la lb)
    (if (empty? la)
      #t
      (if (empty? lb)
        #f
        (if (equal? (head la) (head lb))
          (self self (tail la) (tail lb))
          #f))))
    (prefix?-r prefix?-r la lb))))