Changes to the type checker with respect to UndecidableInstances

[Simon Peyton Jones]

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.

Simon

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

Hi,

I found a difference between GHC 7.6.3 and 7.8.3 with respect to UndecidableInstances.

https://gist.github.com/maoe/57a4346eb36aee159916

7.6.3 requires UndecidableInstances to compile this snippet whereas 7.8.3 doesn't. What has changed in the type checker?

Mitsutoshi

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