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Don't force more than necessary in Data.List.Lazy take #139

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Merged
merged 6 commits into from
May 22, 2018
Merged

Don't force more than necessary in Data.List.Lazy take #139

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25dce0e
Fix bug of Data.List.Lazy(take).
ibara1454 Jan 3, 2018
768708f
Resolve #138
ibara1454 Jan 3, 2018
e1b0b94
Rewrite with pointfree style instead
ibara1454 Jan 20, 2018
803e5e0
Add test case. `take` should evaluate exactly n items which we needed
ibara1454 Jan 20, 2018
13adcea
Replace fromFoldable with l
ibara1454 Jan 20, 2018
8229f7e
Rewrite test case
ibara1454 Jan 20, 2018
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5 changes: 3 additions & 2 deletions src/Data/List/Lazy.purs
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Original file line number Diff line number Diff line change
Expand Up @@ -509,10 +509,11 @@ slice start end xs = take (end - start) (drop start xs)
-- |
-- | Running time: `O(n)` where `n` is the number of elements to take.
take :: forall a. Int -> List a -> List a
take n = List <<< map (go n) <<< unwrap
take n = if n <= 0
then const nil
else List <<< map (go n) <<< unwrap
where
go :: Int -> Step a -> Step a
go i _ | i <= 0 = Nil
go _ Nil = Nil
go n' (Cons x xs) = Cons x (take (n' - 1) xs)

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6 changes: 6 additions & 0 deletions test/Test/Data/List/Lazy.purs
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Expand Up @@ -270,6 +270,12 @@ testListLazy = do
assert $ (take 2 (l [1, 2, 3])) == l [1, 2]
assert $ (take 1 nil') == nil'

log "take should evaluate exactly n items which we needed"
assert let oops x = 0 : oops x
xs = 1 : defer oops
in take 1 xs == l [1]
-- If `take` evaluate more than once, it would crash with a stack overflow

log "takeWhile should keep all values that match a predicate from the front of an list"
assert $ (takeWhile (_ /= 2) (l [1, 2, 3])) == l [1]
assert $ (takeWhile (_ /= 3) (l [1, 2, 3])) == l [1, 2]
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