Here is a small example which shows the problem<br><br>{-# LANGUAGE TypeFamilies #-}<br>{-# LANGUAGE ConstraintKinds #-}<br>{-# LANGUAGE AllowAmbiguousTypes #-}<br><br>module TypeFamilyTest where<br><br>import GHC.Prim<br><br>type family A ∷ * → Constraint<br><br>type family C f g a where C f g a = f (g a)<br><br>a ∷ A (f (g a)) ⇒ ()<br>a = ()<br><br>b ∷ A (C f g a) ⇒ ()<br>b = a<br><br><br>On Wednesday, August 13, 2014 6:54:45 PM UTC-7, Richard Eisenberg wrote:<blockquote class="gmail_quote" style="margin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;">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.
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<br>Richard
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<br>On Aug 13, 2014, at 8:02 PM, Ian Milligan &lt;<a href="javascript:" target="_blank" gdf-obfuscated-mailto="fku5x62a7v4J" onmousedown="this.href='javascript:';return true;" onclick="this.href='javascript:';return true;">ianm...@gmail.com</a>&gt; wrote:
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<br>&gt; 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
<br>&gt; type family (:.:) f g a where (:.:) f g a = f (g a)
<br>&gt; I get errors such as
<br>&gt; 'Could not deduce (Object c3 ((:.:) f g a) ~ Object c3 (f (g a)))'
<br>&gt; (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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