Here's an example: f x = do { "c" ; putStrLn "x"; return () } Test.hs:3: Couldn't match `[]' against `IO' Expected type: [t] Inferred type: IO () In the application `putStrLn "x"' in a `do' expression pattern binding: putStrLn "x" I don't see an easy way to improve this. I suppose we could do some kind of "voting" thing, where we take a majority view about what the result type constructor of the monad is... but that would take a bit of work. Hmm. It's a good point, but I'm inclined to wait until there are more yells before investing effort here. Simon | -----Original Message----- | From: George Russell [mailto:ger@tzi.de] | Sent: 16 July 2001 15:56 | To: glasgow-haskell-users@haskell.org | Subject: do's and error messages | | | If you have a type error message at the very start of a "do" | statement, the result can be rather confusing, because the | typechecker doesn't "know" that do's are almost always have | type (IO (something)) and so tries to shoehorn the monad to | fit the type. This has actually happened to me several | times; the worst case is when the result is (for example) a | list, which allows the error to propagate further. Is there | some way of doing something about this? | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |