[Haskell-cafe] Type synonym involing quantified constraint?
sperber at deinprogramm.de
Tue Aug 31 15:45:32 UTC 2021
I'm working on Conal's ConCat code, and keep putting quantified
constraints like this in instance constraints:
(okk ~ Ok k, forall x y. (okk x, okk y) => okk (Prod k x y))
(The okk ~ Ok k is necessary because Ok is an associated type.)
Is there any way to define a type synonym for this? Like
type OkProd k = (okk ~ Ok k, forall x y. (okk x, okk y) => okk (Prod k x y))
ghc (8.10.4) complains about an impredicative type.
Help would be much appreciated!
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