Sum types with aliases

[dfrese]

Defining sum types does not work when aliasing types or predicates in match expressions, in ClojureScript at least:

(require '[active.clojure.sum-type :as st :include-macros true])
(require '[active.clojure.cljs.record :as rec :include-macros true])

(rec/define-record-type T {:rtd-record? true} (make-t) t? [])
(st/define-sum-type S1 s? [T])
(def S2 S1)
(st/define-sum-type S2 s? [T])
(def T2 T)
(st/define-sum-type S2 s? [T2])

Compile Exception: rtd-record or sum-type required, found: T2

A similar error is raised when using an alias of t? in a match expression.

This is quite a burden for code reuse/abstraction, and should be fixed if possible.

[smoes]

I'm not sure if this is possible. These checks are done at macro-expansion time, how would we handle

(def pred true)
(def T2 (if pred T0 T1))

Various languages offer a function for type-aliasing. Maybe we'd need something like:

(st/defalias T2 T)

Could you provide an example/a rationale why this currently is a burden for code reuse/abstraction?

[dfrese]

Your "ELM-style" architecture in the Siemens-Client requires a page package to provide a State type, and I wanted to define two almost identical pages by specializing an "abstract page" - delegating what the State type actually is to that package.

Can be worked around to some extent by defining both concrete pages in the same namespace.

[dfrese]

More detailed example:

(ns utility)
(rec/define-record-type T {:rtd-record? true} (make-t) t? [])

(ns page-1
  (:require [utility :as utility)
(def T utility/T)

(ns home-page
  (:require [page-1 :as page-1]))

(st/define-sum-type State s? [page-1/T])

I wanted to keep the use of the 'utility' module an implementation detail of module 'page-1'.

[smoes]

With this example, I can see why it would be useful. However, we have to introduce a specific macro for that, this cannot be achieved by def afaik.

[smoes]

We also have to implement that type-alias operation for records.