It seems to be an instance of https://ghc.haskell.org/trac/ghc/ticket/7869
But it is fixed (both in HEAD and 7.8). Probably the fix is partial? On Thu, 2014-12-04 at 14:53 -0500, Richard Eisenberg wrote: > 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 <[email protected]> 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 > > [email protected] > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > > > _______________________________________________ > Glasgow-haskell-users mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users _______________________________________________ Glasgow-haskell-users mailing list [email protected] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
