282: Missing R7RS (Type) Predicates282: Missing R7RS (Type) Predicatesby Artyom Bologov
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Types and entities deserve predicates so that their user can meaningfully understand what they are dealing with, and usefully dispatch on them in their code. However, some R7RS “types” have no respective predicates. This SRFI defines them.
??? Optional section that may point out things to be resolved. This will not appear in the final SRFI.
The discussion of types in Scheme is often predicate-reliant, including in the in R7 “Disjointness of types” section. Types are reliably represented by these predicates. (This is, in part, why the author’s own SRFIs rely on predicates for typing.) The predicate set essentially defines what types of object semantically “exist.” If there’s no predicate, there is no (programmatically-reachable) type.
Thus more predicates are needed than R7RS Small provides. There are types that do actually exist, but have no predicates proving their existence. Some implementations provide these predicates, but there’s no portable way to use them. Thus this SRFI: to have a common ground set of predicates for entities that otherwise exist in Scheme.
There are only four exported procedures in this SRFI.
This predicate returns true whenever given a library object, and false otherwise.
While libraries are part of the “meta-language” of Scheme, they are semantic entities. They might as well be represented in the run-time. Modules are present in many implementations, or otherwise, so there’s a need for a predicate checking for them.
Returns true whenever given an environment object, as returned by environment and the like, and false otherwise.
Lack of a predicate for environments is an interesting omission in the standard. These are first-class entities of a distinct type. They deserve a predicate.
Returns true whenever object is a parameter object, and false otherwise.
While parameters are almost invariably represented as procedures, they might happen to be distinct callable objects. Thus the need for a predicate.
Returns true whenever object is a syntax transformer, like the ones returned by syntax-rules. Returns false otherwise.
While, again, these objects are not necessarily representable in all implementations, being essentially part of the Scheme meta-language some implementations do represent them as “macro” and “syntax” objects reachable programmatically, so a predicate is necessary.
record? and record-type?.
While records and their types are also part of the standard type repertoire,
they have respective SRFIs,
like SRFI-237
and SRFI-76.
Sample implementation not provided yet.
The simplest conforming implementation might be:
(define (library? object) #f) (define (environment? object) #f) (define (parameter? object) #f) (define (syntax-transformer? object) #f)
Thanks to numerous individuals that contributed and continue to contribute to R7RS. This SRFI is by no means an attack on them, but rather a suggestion highlighting the lack of predicates for otherwise important entities.
Thanks to Arthur Gleckler for shepherding SRFI process all these years.
© 2026 Artyom Bologov.
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