Based on Asumu's work on type expanders
(https://github.com/racket/racket/compare/master...takikawa:tr-type-expander),
I have written a library which adds support for type expanders in typed/racket.
Type expanders are to types what match expanders are to match patterns. A type
expander is a special macro which can appear wherever a type would normally be
expected, and must expand to a type.
The library is written using literate programming, with (a variant of)
scribble/lp2. The annotated source code is available here:
http://docs.racket-lang.org/type-expander.hl/
and the documentation here:
http://docs.racket-lang.org/type-expander/
This library comes along with the multi-id library
(http://docs.racket-lang.org/multi-id/, literate source:
http://docs.racket-lang.org/multi-id.hl/), which allows easy definition of an
identifier which acts as a type expander, match expander, normal macro and
identifier macro. It can be used to define new datatypes, where a single
identifier represents the type, match clause, literal
instance as in (foo-datatype 1 2 3), and constructor function as in (map
foo-datatype …).
The type-expander library is in beta status:
* It works well and I have been using it for a while without issues.
* The API should not change significantly in the future.
* However, there are many typed/racket primitives which are not
overloaded yet, as noted in the documentation (patches are welcome).
* Also, a couple of features are missing (patches are welcome again):
* The special form `Let`, which acts like `let-syntax` for types,
is only partially implemented.
* The special form `(Λ (stx) . body)`, which acts as an anonymous
type expander, is not implemented at all.
* There is no syntax-local-type-introduce.
Any feedback is appreciated! :)
Regards,
Georges Dupéron
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