== SRFI-137: Minimal Unique Types

This SRFI provides a simple hook to create new data types at run time
that are disjoint from all existing types.  Subtypes can be created
from any existing type created with this library.

[[toc:]]

== SRFI Description

This page includes excerpts from
[[https://srfi.schemers.org/srfi-137/srfi-137.html|SRFI 137: Minimal Unique Types]].
For further information on the design and rationale of this library,
see the SRFI document.

== Specification

<procedure>(make-type type-payload) → type-accessor constructor predicate accessor make-subtype</procedure>

Calling {{make-type}} on {{type-payload}}, which can be any Scheme object,
returns five values, all of which are procedures.  They are distinct
(in the sense of {{eqv?}}) from each other and from any other procedures
returned by other calls to {{make-type}}.  In brief, the five
functions respectively:

* return {{type-payload}}

* return newly allocated objects of a disjoint type known as instances, each associated with an instance payload

* return {{#t}} iff an object is an instance of this type

* return the instance payload

* return five more procedures associated with a subtype of this type

Details are given for a sample type in the next section.  The type
payload might contain metadata (such as field names or class
variables) associated with the type as a whole.

=== Sample procedures for a type

For the purposes of this section, we will suppose that

<enscript highlight="scheme">(define-values (reia-metadata make-reia reia? reia-ref make-reia-subtype) (make-type 'reia))</enscript>

has been evaluated, and document each of the five variables that it
binds.  "Reia" is an acronym for "remarkably 'evil' in appearance",
and has no particular significance.  Fnord!

<procedure>(reia-metadata) → object</procedure>

Returns the symbol {{reia}}.

<procedure>(make-reia instance-payload)→ reia</procedure>

Returns a newly allocated instance associated with instance-payload.
This association is single and immutable, but it is possible to make
use of an appropriate container payload in order to effectively
associate the instance with more than one value.  To make the
association effectively mutable, use a mutable payload such as a box,
list or vector.  Instances belong to a type that is disjoint from any
existing Scheme type, including types created by other calls to
{{make-type}}.

<procedure>(reia? object)→ boolean</procedure>

Returns {{#t}} iff object was returned by a call to {{make-reia}} or
any constructor created as part of a direct or indirect subtype of the
{{reia}} type.

<procedure>(reia-ref reia) → object</procedure>

Returns the instance payload of {{reia}}.  It is an error if {{reia}}
does not satisfy {{reia?}}.

<procedure>(make-reia-subtype type-payload)→ type-accessor constructor predicate accessor make-subtype</procedure>

Returns five new procedures with the same semantics as {{make-type}},
such that the objects returned by constructor satisfy {{reia?}} and
their payload can be accessed using {{reia-ref}}.

== Implementation

The implementation of SRFI 137 provided by this egg is built on
top of {{define-record-type}}.  Thus, instances are records, and
all of CHICKEN's [[/records-and-oop|existing record-type sugar]]
can be used with SRFI 137 types.

== About This Egg

=== Authors

SRFI 137 was written and shepherded by John Cowan.  The implementation
provided by this egg was created by Marc Nieper-Wißkirchen.

Ported to CHICKEN Scheme 5 by Sergey Goldgaber.

=== Maintainer

Wolfgang Corcoran-Mathe

Contact: {{<wcm at sigwinch dot xyzzy minus the zy>}}

=== Repository

[[https://github.com/Zipheir/srfi-137|GitHub]]

=== Version history

; [[https://github.com/diamond-lizard/srfi-137/releases/tag/0.1|0.1]] : Ported to Chicken Scheme 5
; 0.2 : (2022-01-07) Change maintainer info, port tests to [[test]] egg.
; 0.3 : (2026-08-16) Add CHICKEN 6 support.

=== Copyright

Copyright (C) John Cowan, Marc Nieper-Wißkirchen (2016).  All Rights
Reserved.

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.