ANNOUNCE: GHC 7.0.1 Release Candidate 1

[Don Stewart]

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 at haskell.org [mailto:glasgow-haskell-users-
> |  bounces at haskell.org] On Behalf Of David Fox
> |  Sent: 29 September 2010 04:11
> |  To: glasgow-haskell-users at 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.)
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