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

[[tags: egg]]

== defstruct

[[toc:]]

=== Description

A more convenient form of {{define-record}}

=== Author

Dorai Sitaram, ported to CHICKEN by [[/users/felix winkelmann|felix winkelmann]], with improvements by [[/users/peter-bex|Peter Bex]]

=== Requirements

This only needs the [[srfi-1]] egg.

=== Documentation

<macro>(defstruct NAME SLOT ...)</macro>

Defines a record type with the name {{NAME}}. {{SLOT}} may be a symbol
or a list of the form {{(NAME INIT)}} where {{INIT}} is the default
value of the slot (the {{INIT}} expression will only be evaluated when
no value is given in the constructor procedure. It defaults to {{#f}}
when none is provided).

The {{defstruct}} macro expands into predicate and accessor functions
(just like the native {{define-record}}):

<enscript highlight="scheme">
(import defstruct)

(defstruct point x y)

;; Creates code equivalent to the following:
(begin
  (define (point? x) ...)          ; is x a point?
  (define (point-x p) ...)         ; return x slot of point p
  (define (point-x-set! p n) ...)  ; change x slot of point p to n
  ...)
</enscript>

Additionally, a constructor procedure {{make-STRUCTNAME}} is defined,
which accepts initialization values for all slots specified as keyword
arguments:

<enscript highlight="scheme">
(define p1 (make-point x: 99 y: 42))
 => ; a point with x = 99 and y = 42
</enscript>

If keywords for some slots are missing, their initialization values
default to those supplied in the {{defstruct}} call, or else {{#f}}.

There are also two procedures {{update-STRUCTNAME}} and
{{set-STRUCTNAME!}} defined for functionally and destructively
updating selected values in an existing record:

<enscript highlight="scheme">
(define p2 (update-point p1 x: 100))
p2
 => ; a point with x = 100 and y = 42

(= p1-x 99)
 => #t

(set-point! p1 x: 100)
(= p1-x 100)
 => #t
</enscript>

Finally, there are two conversion procedures for converting to and
from alists, {{STRUCTNAME->alist}} and {{alist->STRUCTNAME}}:

<enscript highlight="scheme">
(define p3 (alist->point '((x . 1) (y . 2))))
p3
 => ; a point with x = 1 and y = 2

(define p4 (make-point x: 123 y: 456))
(point->alist p4)
 => ((x . 123) (y . 456))
</enscript>

=== Bugs and limitations

This macro is unhygienic which, while resulting in concise syntax, has
some drawbacks:

* 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!}}.
* 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.
* For the same reason, you should avoid defining record types named {{make}} or {{update}}.
* You cannot use a field named {{>alist}}.

=== License

  Copyright (c) 2005, Dorai Sitaram
  Copyright (c) 2005, Felix Winkelmann (Chicken port)
  Copyright (c) 2008-2018, Peter Bex (Hygienic Chicken port + extensions)
  All rights reserved.
  
  Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following
  conditions are met:
  
    Redistributions of source code must retain the above copyright notice, this list of conditions and the following
      disclaimer. 
    Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
      disclaimer in the documentation and/or other materials provided with the distribution. 
    Neither the name of the author nor the names of its contributors may be used to endorse or promote
      products derived from this software without specific prior written permission. 
  
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS
  OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
  AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  POSSIBILITY OF SUCH DAMAGE.