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Add flattenSCC1 : SCC vertex -> Data.List.NonEmpty.NonEmpty vertex #985

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andreasabel opened this issue Jan 31, 2024 · 2 comments · May be fixed by #987
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Add flattenSCC1 : SCC vertex -> Data.List.NonEmpty.NonEmpty vertex #985

andreasabel opened this issue Jan 31, 2024 · 2 comments · May be fixed by #987

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@andreasabel
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andreasabel commented Jan 31, 2024 •
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The flattenSCC function throws away the knowledge that a SCC is a non-empty list of vertices:

containers/containers/src/Data/Graph.hs

Lines 248 to 251 in a4e8d16

-- | The vertices of a strongly connected component.
flattenSCC :: SCC vertex -> [vertex]
flattenSCC (AcyclicSCC v) = [v]
flattenSCC (NECyclicSCC vs) = NE.toList vs

In course of the new x-partial warning of GHC 9.8, this is a bit inconvenient, since GHC will now cry about head . flattenSCC and similar.

The more correct type of flattenSCC is SCC vertex -> Data.List.NonEmpty.NonEmpty vertex, but in order not to break everyone's code I suggest to add a new function flattenSCC1 instead.

Should this maybe have gone into containers-0.7?
@Bodigrim
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@andreasabel one workaround is to use Data.Foldable1.toNonEmpty, there is an instance.

@treeowl
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treeowl commented Feb 1, 2024

I don't have a strong opinion either way. I'd lean toward adding the function because it seems a bit odd to give the less informative version its own name but not the more informative one.

andreasabel added a commit that referenced this issue Feb 2, 2024
@andreasabel
Add flattenSCC1 :: SCC a -> NonEmpty a
80179ab 
This gives a more precise type to the existing `flattenSCC :: SCC a -> [a]`.
Closes #985.
@andreasabel andreasabel linked a pull request Feb 2, 2024 that will close this issue
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andreasabel added a commit that referenced this issue Feb 2, 2024
@andreasabel
Add flattenSCC1 :: SCC a -> NonEmpty a
fd3312c 
This gives a more precise type to the existing `flattenSCC :: SCC a -> [a]`.
Closes #985.
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Add flattenSCC1 :: SCC a -> NonEmpty a haskell/containers
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@andreasabel @treeowl @Bodigrim