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FD2.hs:26:34: error: [GHC-25897]
• Couldn't match expected type ‘e1’ with actual type ‘e’
‘e1’ is a rigid type variable bound by
the type signature for:
mf :: forall e1. Elem a e1 => e1 -> Maybe e1 -> Maybe e1
at FD2.hs:24:12-54
‘e’ is a rigid type variable bound by
the type signature for:
foldr1 :: forall e. Elem a e => (e -> e -> e) -> a -> e
at FD2.hs:21:13-47
• In the first argument of ‘Just’, namely ‘(f x y)’
In the expression: Just (f x y)
In an equation for ‘mf’: mf x (Just y) = Just (f x y)
• Relevant bindings include
y :: e1 (bound at FD2.hs:26:23)
x :: e1 (bound at FD2.hs:26:15)
mf :: e1 -> Maybe e1 -> Maybe e1 (bound at FD2.hs:25:12)
f :: e -> e -> e (bound at FD2.hs:22:10)
foldr1 :: (e -> e -> e) -> a -> e (bound at FD2.hs:22:3)
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