Dear Cafe, I recently came upon this little "type-inference puzzle". Can anyone shine some light on why the third example doesn't compile? ``` {-# LANGUAGE TypeApplications #-} -- doesn't work -- I expect this, not sure why it happens {- class C m where c :: () -> m instance C Int where c () = 0 instance C Bool where c () = False f :: (Int, Bool) f = let x = c in (x (), x ()) -} -- works -- I expect this, not sure why it happens {- class C m where c :: () -> m instance C Int where c () = 0 instance C Bool where c () = False f :: (Int, Bool) f = let x u = c u in (x (), x ()) -} -- doesn't work -- I don't expect this, not sure why it happens {- class C m where c :: () -> m instance C Int where c () = 0 instance C Bool where c () = False f :: (Int, Bool) f = let x u = c u in (x @Int (), x @Bool ()) -- wtf.hs:54:5: error: -- • Cannot apply expression of type ‘() -> m0’ -- to a visible type argument ‘Int’ -- • In the expression: x @Int () -- In the expression: (x @Int (), x @Bool ()) -- In the expression: let x u = c u in (x @Int (), x @Bool ()) -- | -- 54 | (x @Int (), x @Bool ()) -- | ^^^^^^^^^ -} ``` Cheers, Georgi