[Haskell-cafe] Closed Type Family Simplification

Ian Milligan ianmllgn at gmail.com
Thu Aug 14 00:02:23 UTC 2014

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?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140813/e4541636/attachment.html>

More information about the Haskell-Cafe mailing list