[[tags: egg]]

== type-extensions

[[toc:]]

=== Description

Miscellaneous extensions for CHICKEN's type system.

The source for this egg is available [[http://git.foldling.org/chicken-type-extensions.git|here]].

==== Requirements

* [[/egg/matchable|matchable]]

=== Usage

{{type-extensions}} should be loaded as a compiler extension with the
{{-extend}} (or {{-X}}) flag to {{csc}}:

    $ csc -extend type-extensions <file>

==== API

<syntax>(define-type name)</syntax>

Shorthand for {{(define-type name (struct name))}}.

<syntax>(define-type (name var ...) type)</syntax>

Defines a complex type alias that can be used in place of {{type}}. In
each usage, all instances of {{var}} in {{type}} will be replaced by the
corresponding form from {{(name var ...)}}.

<enscript highlight="scheme">
(define-type (pair-of a) (pair a a))

(: pair (forall (a) (a -> (pair-of a))))
(define (pair x) (cons x x))

(compiler-typecase (pair 1)
  ((pair-of fixnum)
   (print '(pair-of fixnum)))
  (else
   (print 'else)))
</enscript>

As with CHICKEN's built-in {{define-type}} form, type aliases defined
inside a module are not visible outside of that module.

==== Type Syntax

<syntax>(list . type)</syntax>
<syntax>(list type ...)</syntax>

A dotted tail or ellipsis at the end of a {{list}} type form is
shorthand for a sequence of pairs followed by {{(list-of type)}}.

<enscript highlight="scheme">
;; The following types are equivalent:
(define-type a (list fixnum float . number))
(define-type b (list fixnum float number ...))
(define-type c (pair fixnum (pair float (list-of number))))
</enscript>

=== Author

[[/users/evan-hanson|Evan Hanson]]

=== License

Copyright (c) 2014-2018, 3-Clause BSD.