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add some and many for Lazy List #113

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Merged
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Apr 27, 2017
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add some and many for Lazy List #113

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21 changes: 19 additions & 2 deletions src/Data/List/Lazy.purs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,8 +16,8 @@ module Data.List.Lazy
, (..), range
, replicate
, replicateM
-- , some
-- , many
, some
, many
, repeat
, iterate
, cycle
Expand Down Expand Up @@ -91,6 +91,8 @@ module Data.List.Lazy

import Prelude

import Control.Alt ((<|>))
import Control.Alternative (class Alternative)
import Control.Lazy as Z

import Data.Foldable (class Foldable, foldr, any, foldl)
Expand Down Expand Up @@ -160,6 +162,21 @@ replicateM n m
as <- replicateM (n - one) m
pure (cons a as)

-- | Attempt a computation multiple times, requiring at least one success.
-- |
-- | The `Lazy` constraint is used to generate the result lazily, to ensure
-- | termination.
some :: forall f a. Alternative f => Z.Lazy (f (List a)) => f a -> f (List a)
some v = cons <$> v <*> Z.defer (\_ -> many v)
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Is it possible to generate the tail lazily here?

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@safareli safareli Apr 26, 2017 •
edited
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That's what Z.defer is doing here.
Or do you mean something else?

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Yes, I think this is fine. Sorry again :)

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It's fine :p


-- | Attempt a computation multiple times, returning as many successful results
-- | as possible (possibly zero).
-- |
-- | The `Lazy` constraint is used to generate the result lazily, to ensure
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There is no Lazy constraint here.

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Copied it from List.purs. But even tho Lazy constraint isn't used in many, it's used in some, which is used by many, so it's technically true. If want better wording here please let me know and i'll update it in both places

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Sorry, I misread it, please ignore my comment.

-- | termination.
many :: forall f a. Alternative f => Z.Lazy (f (List a)) => f a -> f (List a)
many v = some v <|> pure nil

-- | Create a list by repeating an element
repeat :: forall a. a -> List a
repeat x = Z.fix \xs -> cons x xs
Expand Down