I've created a wiki page to track the common errors when upgrading to GHC 7. http://haskell.org/haskellwiki/Upgrading_packages/Updating_to_GHC_7 Solutions to each problem should be listed here. simonpj:
I've been meaning to write a blog post about this, because it's a significant change. I'll do this when I get home after ICFP. Briefly though:
* If you use -XGADTs or -XTypeFamilies (or -fglasgow-exts, which is deprecated) you get -XMonoLocalBinds, which says that local let/where bindings are not auto-generalised. There's an extensive discussion of the reason for this decision in our paper "Let should not be generalised" and the journal version http://haskell.org/haskellwiki/Simonpj/Talk:OutsideIn
* One of the main times this matters is in situations like foo = runST (bar 5) where bar x = return x Here 'bar' must have a polymorphic type, in order to be mentioned in runST's argument
* The error message does point out that 'bar' is a possible culprit. In David's example: The following variables have types that mention s fill0 :: MBUArr s e -> ST s Int
* The solution is usually to add a type signature for the offending variable (fillO in this case); you can still have polymorphic bindings in let/where, but they need a type signature.
Simon
| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of David Fox | Sent: 29 September 2010 04:11 | To: glasgow-haskell-users@haskell.org | Subject: Re: ANNOUNCE: GHC 7.0.1 Release Candidate 1 | | I'm seeing errors like this in various places, which I guess are | coming from the new type checker: | | Data/Array/Vector/Prim/BUArr.hs:663:3: | Couldn't match type `s' with `s3' | because this skolem type variable would escape: `s3' | This skolem is bound by the polymorphic type `forall s. ST s a' | The following variables have types that mention s | fill0 :: MBUArr s e -> ST s Int | (bound at Data/Array/Vector/Prim/BUArr.hs:669:5) | In the first argument of `runST', namely | `(do { marr <- newMBU n; | n' <- fill0 marr; | unsafeFreezeMBU marr n' })' | | I would love to hear an explanation about what they mean and what to | do about them. (This one is from uvector.) | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
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