Hello glasgow-haskell-users, as you may remember, in GHC survey awkward error messages was named as one of most serious GHC drawbacks. i propose to start collecting examples of bad error messages together with what we want to see in these cases. as first contribution, i've added this text as http://hackage.haskell.org/trac/ghc/ticket/956 the following program main = do putChar 'a' putChar makes this error message:: Couldn't match expected type `IO' against inferred type `(->) Char' Probable cause: `putChar' is applied to too few arguments In the expression: putChar while the 'probable cause' is very helpful, the error message by itself is hard to understand for novices. i propose in these cases to work against known value types. in this case, we know that 'putChar' has type 'Char -> IO ()' and GHC may report smth like this: 'putChar': expression of type 'Char -> IO ()' used in context that requires expression of 'IO ()' type Probably, you've omitted parameter of type 'Char' to this expression comparing this with existing message shows that may be improved: 1) for me, it's still hard to understand what is "expected" and "inferred" means. i guess that for true beginners it's even harder. i prefer to see something more English and less Mathematic - it will be very helpful 2) instead of obscure 'IO' and `(->) Char' types, i strongly prefer to see more pragmatic 'IO ()' and 'Char -> IO ()' ones 3) 'Probable cause' may be more pleasant by telling position and type of skipped argument(s) my second example is almost the same :) main = do putChar putChar 'a' in this case error message is even worser: Couldn't match expected type `(->) Char' against inferred type `IO' Probable cause: `putChar' is applied to too many arguments In the expression: putChar 'a' afaiu, here GHC thought that putChar should have 'IO ()' type. or, it may think that operations in this monad has type 'Char -> IO ()'. anyway, it will be great if GHC will know that operations in monad usually has type 'IO a' or 'm a' and try to use this heuristic when dealing with such errors (different types of statements in 'do' block) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com