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.
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