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Add either and either_ #382

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May 18, 2020
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Add either and either_ #382

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27 changes: 26 additions & 1 deletion hedgehog/src/Hedgehog/Internal/Gen.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,8 @@ module Hedgehog.Internal.Gen (

-- ** Collections
, maybe
, either
, either_
, list
, seq
, nonEmpty
Expand Down Expand Up @@ -217,7 +219,7 @@ import qualified Data.Text.Encoding as Text
import Data.Word (Word8, Word16, Word32, Word64)

import Hedgehog.Internal.Distributive (MonadTransDistributive(..))
import Hedgehog.Internal.Prelude hiding (maybe, seq)
import Hedgehog.Internal.Prelude hiding (either, maybe, seq)
import Hedgehog.Internal.Seed (Seed)
import qualified Hedgehog.Internal.Seed as Seed
import qualified Hedgehog.Internal.Shrink as Shrink
Expand Down Expand Up @@ -1354,6 +1356,29 @@ maybe gen =
, (1 + fromIntegral n, Just <$> gen)
]

-- | Generates either an 'a' or a 'b'.
--
-- As the size grows, this generator generates @Right@s more often than @Left@s.
--
either :: MonadGen m => m a -> m b -> m (Either a b)
either genA genB =
sized $ \n ->
frequency [
(2, Left <$> genA)
, (1 + fromIntegral n, Right <$> genB)
]

-- | Generates either an 'a' or a 'b', without bias.
--
-- This generator generates as many @Right@s as it does @Left@s.
--
either_ :: MonadGen m => m a -> m b -> m (Either a b)
either_ genA genB =
choice [
Left <$> genA
, Right <$> genB
]

-- | Generates a list using a 'Range' to determine the length.
--
list :: MonadGen m => Range Int -> m a -> m [a]
Expand Down