This seems straightforwardly to be a bug, to me. HEAD gives the same behavior you report below. Please post on the bug tracker at https://ghc.haskell.org/trac/ghc/newticket
Thanks! Richard On Dec 4, 2014, at 1:50 PM, Evan Laforge <qdun...@gmail.com> wrote: > 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. > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users