[Haskell-cafe] type error using ghc 6.10.1, not in previous versions
Hi all. Under I have some old code that broke under ghc 6.10.1. Under (6.6.1), 6.8.1 (and I think 6.8.2), this compiles. import Prelude hiding(catch) import Control.Concurrent import Control.Exception(catch,throw,evaluate) async :: IO a -> IO (MVar a) async ioa = do mVar <- newEmptyMVar forkIO $ catch (ioa >>= putMVar mVar) (putMVar mVar . throw) return mVar Under 6.10. I now get a type error. TypeError.hs:13:55: Ambiguous type variable `e' in the constraint: `GHC.Exception.Exception e' arising from a use of `throw' at TypeError.hs:13:55-59 Probable fix: add a type signature that fixes these type variable(s) Prelude> :t catch catch :: IO a -> (IOError -> IO a) -> IO a Prelude> :t Control.Exception.catch Control.Exception.catch :: (GHC.Exception.Exception e) => IO a -> (e -> IO a) -> IO a What changed that causes this to break between 6.8 and 6.10? In fact, I don't even see the type error (the message is of little help). The function throw returns an `a', so it should unify with the required type for the handler. I tried simplifying the problematic line to: forkIO $ catch (ioa >>= putMVar mVar) (error "boom") (error "boom") should also unify with anything. Can anyone suggest other things I can try and perhaps what is going on? Thanks, - Tim
On Mon, Jan 12, 2009 at 07:14:36PM -0800, Tim Bauer wrote:
Hi all. Under I have some old code that broke under ghc 6.10.1. Under (6.6.1), 6.8.1 (and I think 6.8.2), this compiles.
import Prelude hiding(catch) import Control.Concurrent import Control.Exception(catch,throw,evaluate)
async :: IO a -> IO (MVar a) async ioa = do mVar <- newEmptyMVar forkIO $ catch (ioa >>= putMVar mVar) (putMVar mVar . throw) return mVar
Under 6.10. I now get a type error.
TypeError.hs:13:55: Ambiguous type variable `e' in the constraint: `GHC.Exception.Exception e' arising from a use of `throw' at TypeError.hs:13:55-59 Probable fix: add a type signature that fixes these type variable(s)
Prelude> :t catch catch :: IO a -> (IOError -> IO a) -> IO a Prelude> :t Control.Exception.catch Control.Exception.catch :: (GHC.Exception.Exception e) => IO a -> (e -> IO a) -> IO a
What changed that causes this to break between 6.8 and 6.10? In fact, I don't even see the type error (the message is of little help). The function throw returns an `a', so it should unify with the required type for the handler.
Hi Tim, There were messages about this on the list before, I believe.
From ghc 6.10.1 release notes, what changed is this "Control.Exception now uses extensible exceptions. The old style of exceptions are still available in Control.OldException, but we expect to remove them in a future release. " http://www.haskell.org/ghc/docs/6.10.1/html/users_guide/release-6-10-1.html
Providing a type signature for the argument of throw ought to fix it. forkIO $ catch (ioa >>= putMVar mVar) (\e -> (putMVar mVar . throw) (e :: IOException) ) I don't really understand what extensible exceptions are, just that providing a type signature makes the compiler happy. This www.haskell.org/~simonmar/papers/ext-exceptions.pdf might provide the explanation. Hope that helps. Anish
I tried simplifying the problematic line to: forkIO $ catch (ioa >>= putMVar mVar) (error "boom") (error "boom") should also unify with anything.
Can anyone suggest other things I can try and perhaps what is going on? Thanks, - Tim _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (2)
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Anish Muttreja -
Tim Bauer