[Haskell-cafe] Closed Type Family Simplification
ianmllgn at gmail.com
Thu Aug 14 02:54:41 UTC 2014
Actually, that example is not quite right. Let me try to come up with
On Wednesday, August 13, 2014 6:54:45 PM UTC-7, Richard Eisenberg wrote:
> 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.
> > 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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