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Convert a Maybe returning function to Unfold #3188

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Convert a Maybe returning function to Unfold #3188

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21 changes: 21 additions & 0 deletions core/src/Streamly/Internal/Data/Unfold/Type.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,7 @@ module Streamly.Internal.Data.Unfold.Type
, unfoldr
, functionM
, function
, functionMaybeM
, identity

-- * From Values
Expand Down Expand Up @@ -886,6 +887,26 @@ functionM f = Unfold step inject
function :: Applicative m => (a -> b) -> Unfold m a b
function f = functionM $ pure Prelude.. f

-- | Lift a monadic Maybe returning function into an unfold. The unfold
-- generates a singleton stream.
--
{-# INLINE functionMaybeM #-}
functionMaybeM :: Monad m => (a -> m (Maybe b)) -> Unfold m a b
functionMaybeM f = Unfold step inject

where

inject a = return (Just a)

{-# INLINE_LATE step #-}
step (Just a) = do
result <- f a
case result of
Just b -> pure $ Yield b Nothing
Nothing -> pure Stop

step Nothing = pure Stop

-- | Identity unfold. The unfold generates a singleton stream having the input
-- as the only element.
--
Expand Down
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