ANNOUNCE: GHC 7.4.1 Release Candidate 1

[Simon Peyton-Jones]

Yes, it's expected; it's also the behaviour of GHC 6.12 etc.

Here what is happening.  You define
	result = undefined
What type does it get?  In 6.12, and 7.4, it gets type
	result :: forall b. b
So the two uses of 'result' in the two branches of the case have no effect on each other.

But in 7.2 it was *not generalised*, so we got
	result :: f2 a
And now the two uses *do* affect each other.


Why the change. You'll remember that over the last year GHC has changed not to generalise local lets: http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7

I relaxed the rule in 7.2, as discussed in "Which bindings are affected?" in that post. For reasons I have not investigated, 7.2 *still* doesn't generalise 'result'; but 7.4 correctly does.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
| users-bounces at haskell.org] On Behalf Of Antoine Latter
| Sent: 23 December 2011 04:21
| To: glasgow-haskell-users at haskell.org
| Subject: Re: ANNOUNCE: GHC 7.4.1 Release Candidate 1
| 
| On Wed, Dec 21, 2011 at 1:29 PM, Ian Lynagh <igloo at earth.li> wrote:
| >
| > We are pleased to announce the first release candidate for GHC 7.4.1:
| >
| >    http://www.haskell.org/ghc/dist/7.4.1-rc1/
| >
| > This includes the source tarball, installers for OS X and Windows, and
| > bindists for amd64/Linux, i386/Linux, amd64/FreeBSD and i386/FreeBSD.
| >
| > Please test as much as possible; bugs are much cheaper if we find them
| > before the release!
| >
| 
| Hurrah!
| 
| The following used to compile with GHC 7.2.1:
| 
| >>>>>
| {-# LANGUAGE RankNTypes, TypeFamilies, GADTs #-}
| 
| import Data.Typeable ( Typeable1, gcast1, typeOf1 )
| 
| cast1 :: (Typeable1 f1, Typeable1 f2) => f1 a -> f2 a
| cast1 val
|   = case gcast1 (Just val) of
|       Just (Just typed_val) -> typed_val `asTypeOf` result
|       Nothing -> error $ "Invalid cast: " ++ tag ++ " -> " ++ show
| (typeOf1 result)
|   where result = undefined
|         tag = show (typeOf1 val)
| 
| main = putStrLn "Hello, world!"
| <<<<<
| 
| But with GHC 7.4.1 RC 1 I get the error:
| 
| >>>>>
| BugDowncast.hs:9:69:
|     Ambiguous type variable `t0' in the constraint:
|       (Typeable1 t0) arising from a use of `typeOf1'
|     Probable fix: add a type signature that fixes these type variable(s)
|     In the first argument of `show', namely `(typeOf1 result)'
|     In the second argument of `(++)', namely `show (typeOf1 result)'
|     In the second argument of `(++)', namely
|       `" -> " ++ show (typeOf1 result)'
| <<<<<
| 
| Is this an expected change, or should I create a ticket?
| 
| Thanks,
| Antoine
| 
| >
| > The release notes are not yet available, but here are some of the
| > highlights of the 7.4 branch since 7.2 and 7.0:
| >
| >  * There is a new feature Safe Haskell (-XSafe, -XTrustworthy, -XUnsafe):
| >      http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/safe-
| haskell.html
| >    The design has changed since 7.2.
| >
| >  * There is a new feature kind polymorphism (-XPolyKinds):
| >      http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/kind-
| polymorphism-and-promotion.html
| >    A side-effect of this is that, when the extension is not enabled, in
| >    certain circumstances kinds are now defaulted to * rather than being
| >    inferred.
| >
| >  * There is a new feature constraint kinds (-XConstraintKinds):
| >
|  http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/constraint-
| kind.html
| >
| >  * It is now possible to give any sort of declaration at the ghci prompt:
| >
|  http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/interactive-
| evaluation.html#ghci-decls
| >    For example, you can now declare datatypes within ghci.
| >
| >  * The profiling and hpc implementations have been merged and overhauled.
| >    Visible changes include renaming of profiling flags:
| >      http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/flag-
| reference.html#id589412
| >    and the cost-centre stacks have a new semantics, which should in most
| >    cases result in more useful and intuitive profiles. The +RTS -xc flag
| >    now also gives a stack trace.
| >
| >  * It is now possible to write compiler plugins:
| >      http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/compiler-
| plugins.html
| >
| >  * DPH support has been significantly improved.
| >
| >  * There is now preliminary support for registerised compilation using
| >    LLVM on the ARM platform.
| >
| >
| > Note: The release candidate accidentally includes the random, primitive,
| > vector and dph libraries. The final release will not include them.
| >
| >
| > Thanks
| > Ian, on behalf of the GHC team
| >
| >
| > _______________________________________________
| > Glasgow-haskell-users mailing list
| > Glasgow-haskell-users at haskell.org
| > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
| 
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