Re: [Haskell-cafe] Type synonym involing quantified constraint?

This looks like a bug, which I've now filed: https://gitlab.haskell.org/ghc/ghc/-/issues/20318 The workaround is to enable ImpredicativeTypes. Sadly, ImpredicativeTypes is really a bug in GHC 8.10, fixed to work properly in GHC 9.2. So if I were doing this, I would define the offending synonym in a module all by itself so that the scope of ImpredicativeTypes is as small as possible. Richard

On Aug 31, 2021, at 11:45 AM, Michael Sperber wrote:

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!

-- Regards, Mike

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