Changes to the type checker with respect to UndecidableInstances
Simon Peyton Jones
simonpj at microsoft.com
Tue Dec 30 10:56:18 UTC 2014
UndecidableInstances is supposed to be needed if GHC can't prove that the instance declarations terminate. But here it can be sure they terminate. GHC 7.6.3 didn't realise this.
I'll modify the user manual to be clearer on this point.
From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Mitsutoshi Aoe
Sent: 28 December 2014 10:47
To: glasgow-haskell-users at haskell.org
Subject: Changes to the type checker with respect to UndecidableInstances
I found a difference between GHC 7.6.3 and 7.8.3 with respect to UndecidableInstances.
7.6.3 requires UndecidableInstances to compile this snippet whereas 7.8.3 doesn't. What has changed in the type checker?
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