ANNOUNCE: GHC 7.4.1 Release Candidate 1

[Antoine Latter]

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
>
>
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