Continuing roadblocks when trying to learn Haskell.
I am following the "Real World Haskell" and when it comes to getting one of their complex examples to work, I obtain the following error
"Ambiguous type variable e' in the constraint:
GHC.Exception.Exception e'
arising from a use of `handle' at FoldDir.hs:88:14-61
Probable fix: add a type signature that fixes these type variable(s)"
My relevant bits of code are:
import Control.Exception (bracket, handle)
maybeIO :: IO a -> IO (Maybe a)
maybeIO act = handle (\_ -> return Nothing) (Just `liftM` act)
How do I eradicate this error?
You need to specify a type for the handler-function's argument, so it knows which kinds of exceptions to handle.
You can either do this by naming the function
import Control.Exception (handle, SomeException)
maybeIO act = handle handler (Just `liftM` act)
where handler :: SomeException -> IO (Maybe a)
handler _ = return Nothing
Or by using the ScopedTypeVariables
extension:
{-# LANGUAGE ScopedTypeVariables #-}
import Control.Exception (handle, SomeException)
maybeIO act = handle (\(_ :: SomeException) -> return Nothing) (Just `liftM` act)
Control.Exception
exposes a different interface in GHC 6.10 and later. For a quick fix, change
import Control.Exception (bracket, handle)
to
import Control.OldException (bracket, handle)
Control.OldException is deprecated. With extensible exceptions, one needs to specify a type for the exception, even if it is never used. My version of maybeIO is
perhaps ∷ forall a. IO a → IO (Maybe a)
perhaps task = do
result ← (try ∷ IO a → IO (Either IOException a)) task
return $ either (const Nothing) Just result
One needs the explicit forall
to bring the type variable a
into scope, and the GHC compiler flag -XExplicitForAll
to use the explicit forall
. One cannot give try
a type out in the open without getting the error "You cannot give a type signature for an imported value." If one tries the type declaration elsewhere, e.g. on the result of try task
then GHC can't sort it out. So yes, this is a bit embarrassing for us strong typing advocates, but it gets better.
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