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 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
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@haskell.org] On Behalf Of Mitsutoshi Aoe Sent: 28 December 2014 10:47 To: glasgow-haskell-users@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
Hi Simon,
Thanks. So there was a bug in the type checker. Do you have a link to
the ticket or commits which fixed the issue?
Mitsutoshi
2014-12-30 19:56 GMT+09:00 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@haskell.org] On Behalf Of Mitsutoshi Aoe Sent: 28 December 2014 10:47 To: glasgow-haskell-users@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
participants (2)
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Mitsutoshi Aoe -
Simon Peyton Jones