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add nats: natural numbers for lazy unfoldable #12

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add nats: natural numbers for lazy unfoldable #12

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3 changes: 2 additions & 1 deletion bower.json
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Expand Up @@ -18,7 +18,8 @@
],
"dependencies": {
"purescript-partial": "^1.2.0",
"purescript-tuples": "^4.0.0"
"purescript-tuples": "^4.0.0",
"purescript-control": "^3.3.1"
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No need for this any more, right?

},
"devDependencies": {
"purescript-assert": "^3.0.0",
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12 changes: 11 additions & 1 deletion src/Data/Unfoldable.purs
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Expand Up @@ -11,15 +11,16 @@ module Data.Unfoldable
, none
, singleton
, range
, nats
, fromMaybe
) where

import Prelude

import Control.Lazy (class Lazy)
import Data.Maybe (Maybe(..), isNothing, fromJust)
import Data.Traversable (class Traversable, sequence)
import Data.Tuple (Tuple(..), fst, snd)

import Partial.Unsafe (unsafePartial)

-- | This class identifies data structures which can be _unfolded_,
Expand Down Expand Up @@ -94,6 +95,15 @@ range :: forall f. Unfoldable f => Int -> Int -> f Int
range start end =
unfoldr (\i -> if i <= end then Just (Tuple i $ i + 1) else Nothing) start

-- | Produce a Lazy Unfoldable structure representing the natural numbers.
--
-- The result of this function is infinite.
nats :: forall f a. Unfoldable f => Semiring a => Lazy (f a) => f a
nats = unfoldr increment zero
where
increment :: a -> Maybe (Tuple a a)
increment a = Just (Tuple a $ add one a)

-- | Convert a Maybe to any Unfoldable like lists and arrays.
fromMaybe :: forall f a. Unfoldable f => Maybe a -> f a
fromMaybe = unfoldr (\b -> flip Tuple Nothing <$> b)