ANNOUNCE: GHC 7.0.1 Release Candidate 1

[Simon Peyton-Jones]

Thanks.  I have written a blog post to describe the generalisation question and describe how to adapt your code

	http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7

I've added the link to Don's GHC7 page.

Simon

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Don Stewart
|  Sent: 29 September 2010 14:53
|  To: Simon Peyton-Jones
|  Cc: glasgow-haskell-users at haskell.org
|  Subject: Re: ANNOUNCE: GHC 7.0.1 Release Candidate 1
|  
|  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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|  
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