[Haskell-cafe] Closed Type Family Simplification

[Richard Eisenberg]

Your operating assumption sounds right. Do you have a complete, minimal example showing the error? If not, I recommend using -fprint-explicit-kinds to see if kinds are getting in your way at all.

Richard

On Aug 13, 2014, at 8:02 PM, Ian Milligan <ianmllgn at gmail.com> wrote:

> When a closed type family has only one instance it seems like it should never fail to simplify. Yet this doesn't appear to be the case. When I defined (in GHC 7.8.3) the closed type family
> type family (:.:) f g a where (:.:) f g a = f (g a)
> I get errors such as
> 'Could not deduce (Object c3 ((:.:) f g a) ~ Object c3 (f (g a)))'
> (where Object is a Constraint family), indicating that f (g a) is not being substituted for (:.:) f g a as desired. Any idea why this happens?
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