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FD2.hs:26:34:
Could not deduce (e ~ e1)
from the context (Foldable a)
bound by the class declaration for `Foldable'
at FD2.hs:(17,1)-(26,39)
or from (Elem a e)
bound by the type signature for
foldr1 :: Elem a e => (e -> e -> e) -> a -> e
at FD2.hs:21:13-47
or from (Elem a e1)
bound by the type signature for
mf :: Elem a e1 => e1 -> Maybe e1 -> Maybe e1
at FD2.hs:24:18-54
`e' is a rigid type variable bound by
the type signature for
foldr1 :: Elem a e => (e -> e -> e) -> a -> e
at FD2.hs:21:13
`e1' is a rigid type variable bound by
the type signature for
mf :: Elem a e1 => e1 -> Maybe e1 -> Maybe e1
at FD2.hs:24:18
Relevant bindings include
foldr1 :: (e -> e -> e) -> a -> e (bound at FD2.hs:22:3)
f :: e -> e -> e (bound at FD2.hs:22:10)
mf :: e1 -> Maybe e1 -> Maybe e1 (bound at FD2.hs:25:12)
x :: e1 (bound at FD2.hs:26:15)
y :: e1 (bound at FD2.hs:26:23)
In the return type of a call of `f'
In the first argument of `Just', namely `(f x y)'
In the expression: Just (f x y)
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