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-boun...@haskell.org [mailto:glasgow-haskell-
| users-boun...@haskell.org] On Behalf Of Antoine Latter
| Sent: 23 December 2011 04:21
| To: glasgow-haskell-users@haskell.org
| Subject: Re: ANNOUNCE: GHC 7.4.1 Release Candidate 1
|
| On Wed, Dec 21, 2011 at 1:29 PM, Ian Lynagh <ig...@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@haskell.org
| > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
|
| _______________________________________________
| Glasgow-haskell-users mailing list
| Glasgow-haskell-users@haskell.org
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