Lazy List's mapWithIndex is not lazy

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
48/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Domain
backend

Research direction

Start in src/Data/List/Lazy/Types.purs at the linked FunctorWithIndex instance around line 116. Compare its folding implementation with the lazy Functor implementation, then check how the proposed recursive mapping handles the list one step at a time. Done means mapWithIndex preserves laziness while supplying each element’s index; the issue provides no test file to run.

Written by the indexing model from the issue text.

Description

https://github.com/purescript/purescript-lists/blob/b113451e5b41cad87d669a3165f955c71cd863e2/src/Data/List/Lazy/Types.purs#L116

The implementation of FunctorWithIndex for Lazy List uses folding, which, unlike Functor, is not lazy.

It should be:

instance functorWithIndexList :: FunctorWithIndex Int List where
  mapWithIndex f xs = List (go 0 <$> unwrap xs)
    where
    go i = \step -> case step of
      Nil -> Nil
      Cons x xs' -> Cons (f i x) (List (go (i + 1) <$> unwrap xs'))
Dominant language
PureScript
Stars
61
Forks
50
Avg merge
2d 6h
Merged PRs (30d)
1

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