[Haskell-cafe] Control.Exception Funny

[Dominic Steinitz]

I'm probably doing something wrong but this example doesn't compile for
me under ghc 6.10.1
(http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html#4):

>    catch (openFile f ReadMode)
>        (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))

> Run.hs:77:24:
>     Couldn't match expected type `Handle' against inferred type `()'
>       Expected type: IO Handle
>       Inferred type: IO ()
>     In the expression:
>         hPutStr stderr ("Couldn't open " ++ d ++ ": " ++ show e)
>     In the second argument of `CE.catch', namely
>         `(\ e -> hPutStr stderr ("Couldn't open " ++ d ++ ": " ++ show e))'

Fair enough because openFile returns a Handle and hPutStr returns () so
they don't match as the compiler says.

> CE.catch :: (CE.Exception e) => IO a -> (e -> IO a) -> IO a

So if I fix the example thus:

> foo d = CE.catch (openFile d ReadMode >> return ())
>                  (\e -> hPutStr stderr ("Couldn't open "++ d ++": " ++ show e))

I get

> Run.hs:70:8:
>     Ambiguous type variable `e' in the constraint:
>       `CE.Exception e'
>         arising from a use of `CE.catch' at Run.hs:(70,8)-(71,78)
>     Probable fix: add a type signature that fixes these type variable(s)

Now I think I never used to get this under 6.8.2 but I don't easily have
a 6.8.2 to try it out on.

Doing what the compiler suggests doesn't work for obvious reasons:

> foo :: CE.Exception e => FilePath -> IO ()
> foo d = CE.catch (openFile d ReadMode >> return ())
>                  (\e -> hPutStr stderr ("Couldn't open "++ d ++": " ++ show e))

> Run.hs:69:0:
>     Ambiguous constraint `CE.Exception e'
>         At least one of the forall'd type variables mentioned by the constraint
>         must be reachable from the type after the '=>'
>     In the type signature for `foo':
>       foo :: (CE.Exception e) => FilePath -> IO ()

There seems to be a ticket for it
(http://hackage.haskell.org/trac/ghc/ticket/2819) but this doesn't give
a suggested example that compiles.

Dominic.