maybemptcbug.hs

{-# LANGUAGE MultiParamTypeClasses #-}

class C a b where
    idk :: a -> b -> Maybe (a, b)

instance C Int Bool where idk _ _ = Nothing

f :: Int -> Bool -> [Either Int Bool]
f a b =
    let g a b = maybe [] (const $ []) $ idk a b in
    let h thing = g a thing in
    g a b

{-
wtfwtf.hs:11:9:
    Non type-variable argument in the constraint: C Int b
    (Use FlexibleContexts to permit this)
    When checking that ‘h’ has the inferred type
      h :: forall t b. C Int b => b -> [t]
    In the expression: let h thing = g a thing in g a b
    In the expression:
      let g a b = maybe [] (const $ []) $ idk a b in
      let h thing = g a thing in g a b
Failed, modules loaded: none.
-}