source: project/wiki/eggref/5/defstruct @ 39530

Last change on this file since 39530 was 39530, checked in by sjamaan, 3 months ago

defstruct: Clarify that the default value for slots without initializers is #f

File size: 4.4 KB
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1[[tags: egg]]
2
3== defstruct
4
5[[toc:]]
6
7=== Description
8
9A more convenient form of {{define-record}}
10
11=== Author
12
13Dorai Sitaram, ported to CHICKEN by [[/users/felix winkelmann|felix winkelmann]], with improvements by [[/users/peter-bex|Peter Bex]]
14
15=== Requirements
16
17This only needs the [[srfi-1]] egg.
18
19=== Documentation
20
21<macro>(defstruct NAME SLOT ...)</macro>
22
23Defines a record type with the name {{NAME}}. {{SLOT}} may be a symbol
24or a list of the form {{(NAME INIT)}} where {{INIT}} is the default
25value of the slot (the {{INIT}} expression will only be evaluated when
26no value is given in the constructor procedure. It defaults to {{#f}}
27when none is provided).
28
29The {{defstruct}} macro expands into predicate and accessor functions
30(just like the native {{define-record}}):
31
32<enscript highlight="scheme">
33(import defstruct)
34
35(defstruct point x y)
36
37;; Creates code equivalent to the following:
38(begin
39  (define (point? x) ...)          ; is x a point?
40  (define (point-x p) ...)         ; return x slot of point p
41  (define (point-x-set! p n) ...)  ; change x slot of point p to n
42  ...)
43</enscript>
44
45Additionally, a constructor procedure {{make-STRUCTNAME}} is defined,
46which accepts initialization values for all slots specified as keyword
47arguments:
48
49<enscript highlight="scheme">
50(define p1 (make-point x: 99 y: 42))
51 => ; a point with x = 99 and y = 42
52</enscript>
53
54If keywords for some slots are missing, their initialization values
55default to those supplied in the {{defstruct}} call, or else {{#f}}.
56
57There are also two procedures {{update-STRUCTNAME}} and
58{{set-STRUCTNAME!}} defined for functionally and destructively
59updating selected values in an existing record:
60
61<enscript highlight="scheme">
62(define p2 (update-point p1 x: 100))
63p2
64 => ; a point with x = 100 and y = 42
65
66(= p1-x 99)
67 => #t
68
69(set-point! p1 x: 100)
70(= p1-x 100)
71 => #t
72</enscript>
73
74Finally, there are two conversion procedures for converting to and
75from alists, {{STRUCTNAME->alist}} and {{alist->STRUCTNAME}}:
76
77<enscript highlight="scheme">
78(define p3 (alist->point '((x . 1) (y . 2))))
79p3
80 => ; a point with x = 1 and y = 2
81
82(define p4 (make-point x: 123 y: 456))
83(point->alist p4)
84 => ((x . 123) (y . 456))
85</enscript>
86
87=== Bugs and limitations
88
89This macro is unhygienic which, while resulting in concise syntax, has
90some drawbacks:
91
92* Just like with {{define-record}} you cannot use field names ending with {{-set!}} if you also have a similar field name not ending with {{-set!}}.
93* Just like with {{define-record}} you should avoid defining record types with a name that is a prefix of another record type's name in the same module, because that might cause naming clashes.
94* For the same reason, you should avoid defining record types named {{make}} or {{update}}.
95* You cannot use a field named {{>alist}}.
96
97=== License
98
99  Copyright (c) 2005, Dorai Sitaram
100  Copyright (c) 2005, Felix Winkelmann (Chicken port)
101  Copyright (c) 2008-2018, Peter Bex (Hygienic Chicken port + extensions)
102  All rights reserved.
103 
104  Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following
105  conditions are met:
106 
107    Redistributions of source code must retain the above copyright notice, this list of conditions and the following
108      disclaimer.
109    Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
110      disclaimer in the documentation and/or other materials provided with the distribution.
111    Neither the name of the author nor the names of its contributors may be used to endorse or promote
112      products derived from this software without specific prior written permission.
113 
114  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS
115  OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
116  AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
117  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
118  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
119  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
120  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
121  OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
122  POSSIBILITY OF SUCH DAMAGE.
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