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:[email protected]] On Behalf Of Mitsutoshi Aoe Sent: 28 December 2014 10:47 To: [email protected] 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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