Defining a seemingly simple Foldable instance
Question
What would the Foldable instance for this datatype look like?
data X t = X t [X t]
I tried this:
instance Foldable X where
foldMap f (X x xs) = f x `mappend` foldMap f xs
But got this error:
Occurs check: cannot construct the infinite type: a = X a
When generalising the type(s) for `foldMap'
In the instance declaration for `Foldable X'
Solution
xs
is a list of items and foldMap
needs to be applied to the individual items, not the list itself. Doing this with map
gives a list of results that can be combined with mconcat
:
instance Foldable X where
foldMap f (X x xs) = f x `mappend` mconcat (map (foldMap f) xs)
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