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Add a function in Control.Lens.Plated to fold multiple plated things at once? #776

benjamin-hodgson opened this Issue Dec 11, 2017 · 0 comments


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benjamin-hodgson commented Dec 11, 2017

I sometimes find the following generic function useful:

zipPara :: Plated a => ([a] -> [r] -> r) -> [a] -> r
zipPara f xs = f xs $ zipPara f <$> alaf ZipList traverse (toListOf plate) xs

It's like para, except it consumes multiple as at once, zipping together the children from each a's plate. So the list of rs that gets passed to the function is the length of the shortest plate in xs.

Useful for, eg, testing two trees for equality, if for whatever reason you don't want to use the derived ==:

data Expr = Lit Int | Add Expr Expr

eq :: Expr -> Expr -> Bool
eq x y = zipPara (\[x, y] children -> and (eqOneLayer x y : children)) [x, y]
        eqOneLayer (Lit x) (Lit y) = x == y
        eqOneLayer (Add _ _) (Add _ _) = True
        eqOneLayer _ _ = False

Would you guys be interested in a PR to add (a suitably optimised version of) this function (and a zipParaOf sibling) to the Plated module?

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