Today I tried to reduce a list of functions trough monoid typeclass but the resulting function expects its argument to be an instance of Monoid for some reason.
GHCI tells me that the type of mconcat [id, id, id, id]
is Monoid a => a -> a
. Yet I would expect it to be a -> a
.
What is happening?
You're using this instance:
instance Monoid b => Monoid (a -> b) where
mempty _ = mempty
mappend f g x = f x `mappend` g x
which is more general because it doesn't require endomorphisms (ie a -> a
). To get the instance you were expecting, you can wrap your functions in Endo
:
appEndo (mconcat [Endo id, Endo id, Endo id, Endo id])
or
appEndo $ mconcat $ fmap Endo [id, id, id, id]
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