I recently got a confusing error msg, and reduced it to a small case: f1 :: Monad m => m Bool f1 = f2 0 0 'a' f2 :: Monad m => Int -> Float -> m Bool f2 = undefined
From this, it's clear that f2 is being given an extra Char argument it didn't ask for. However, the error msg (ghc 7.8.3) is:
Couldn't match type ‘m Bool’ with ‘Bool’ Expected type: Char -> m Bool Actual type: Char -> Bool Relevant bindings include f1 :: m Bool (bound at Bug.hs:4:1) The function ‘f2’ is applied to three arguments, but its type ‘Int -> Float -> Char -> Bool’ has only three In the expression: f2 0 0 'a' In an equation for ‘f1’: f1 = f2 0 0 'a' The confusing part is that 'f2' was applied to three arguments, but it's type has only three. It includes the Char in expected and actual types, and implies that the type of 'f2' includes the Char. So I took quite a while to realize that the type of 'f2' in fact *didn't* expect a Char (and had an 'm'), so that the "but its type" is *not* in fact its declared type. I suppose it infers a type for 'f2' based on its use, and that then becomes the "actual" type, but it seems less confusing if it picked the declared type of 'f2' as its actual type. Perhaps this is working as intended, but it it is confusing! Especially the part about "expected three but got three". Ideally I'd like to see "too many arguments" or at least "expected (Char -> m Bool) but actually 'm Bool'". Actually I'd expect the other way: "expected 'm Bool' but got (Char -> m Bool)' but I think ghc has always done it backwards from how I expect. It looks like it's substituting (->) for 'm', so maybe it's one of those things where ((->) a) is also a monad.