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fix alt for ZipList #150

Merged
merged 2 commits into from
Dec 26, 2020
Merged

fix alt for ZipList #150

merged 2 commits into from
Dec 26, 2020

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xgrommx
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@xgrommx xgrommx commented Jul 31, 2018

No description provided.

@garyb
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garyb commented Jul 31, 2018

Seems reasonable to me. Anyone else?

natefaubion
natefaubion reviewed Jul 31, 2018
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src/Data/List/ZipList.purs Outdated
@@ -46,7 +47,7 @@ instance applicativeZipList :: Applicative ZipList where
pure = ZipList <<< repeat

instance altZipList :: Alt ZipList where
alt = append
alt (ZipList xs) (ZipList ys) = ZipList $ (xs <> drop (length xs) ys)
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Unnecessary $, or unnecessary parens.

@garyb garyb added the type: breaking change A change that requires a major version bump. label Nov 11, 2018
@hdgarrood
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Can you describe what is being fixed here? Also, do you have a proof that this version satisfies all the relevant laws?

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Also won’t this diverge if the first argument is infinite?

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hdgarrood commented May 7, 2019 •
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Ah ok, so I presume this is motivated by the fact that the current version doesn’t satisfy the Alternative distributivity law

(f <|> g) <*> x == (f <*> x) <|> (g <*> x)

since with the current instance, the left hand side will be a list of length min(length f + length g, x), whereas the right hand side will be a list of length min(length f, length x) + min(length g, length x). For example, with f = g = ZipList ((_+1) : nil) and x = ZipList (0 : nil) we get the left hand side being ZipList (1 : nil) but the right hand side being ZipList (1 : 1 : nil).

@hdgarrood hdgarrood mentioned this pull request May 25, 2020
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@kl0tl kl0tl mentioned this pull request Dec 19, 2020
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I'm going to merge this momentarily, just as soon as I've written a satisfactory commit message

@hdgarrood hdgarrood merged commit 0550216 into purescript:master Dec 26, 2020
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@natefaubion natefaubion natefaubion left review comments

@LiamGoodacre LiamGoodacre LiamGoodacre approved these changes

@hdgarrood hdgarrood hdgarrood approved these changes

@thomashoneyman thomashoneyman thomashoneyman approved these changes

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@xgrommx @garyb @hdgarrood @natefaubion @LiamGoodacre @thomashoneyman