foldr': lazy right folds #118
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src/Data/List/Lazy.purs
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| @@ -705,3 +706,11 @@ foldM f a xs = | |||
| Nothing -> pure a | |||
| Just { head: b, tail: bs } -> | |||
| f a b >>= \a' -> foldM f a' bs | |||
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| -- | Perform a right fold lazily | |||
| foldr' :: forall a b. Z.Lazy b => (a -> b -> b) -> b -> List a -> b | |||
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Can this be written with regular foldr?
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Well, something like this will compile:
foldr' op = foldr op'
where op' a b = Z.defer \_ -> a `op` bBut using foldr kills the laziness. So the test, for example, will just freeze on its infinite input.
Thinking if there is any kind of trick to be done here.
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Honestly, given the definition of foldr as foldl (strict) on the reversed list, and the initial impetus for this change in #111 , I'm at a loss for how such a thing could be done.
Would love to know if you have something in mind.
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I wasn't sure if it was possible, but thanks for trying. This looks good!
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Ah, I thought you knew of a way and were attempting to deliver it from me Socratically :p
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Could you please rebase this? Thanks! |
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Also can this be called |
addresses purescript#111
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rebased and renamed |
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Thanks! |
addresses #111
This looks a bit different from the signature discussed in that issue, but my inclination is that this is what we are actually looking for. Happy to discuss better ways if anyone can think of one, though.