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.
This page includes excerpts from SRFI 137: Minimal Unique Types. For further information on the design and rationale of this library, see the SRFI document.
- make-type type-payloadprocedure
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
(define-values (reia-metadata make-reia reia? reia-ref make-reia-subtype) (make-type 'reia))
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!
Returns the symbol reia.
- make-reia instance-payloadprocedure
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.
- reia? objectprocedure
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.
- reia-ref reiaprocedure
Returns the instance payload of reia. It is an error if reia does not satisfy reia?.
- make-reia-subtype type-payloadprocedure
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.
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 existing record-type sugar can be used with SRFI 137 types.
About This Egg
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.
Contact: <wcm at sigwinch dot xyzzy minus the zy>
Copyright (C) John Cowan, Marc Nieper-Wißkirchen (2016). All Rights Reserved.
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