On Wed, Dec 21, 2011 at 1:29 PM, Ian Lynagh
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.ht... 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-polymorphi... 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...
* 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-eva... 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.... 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-plugin...
* 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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