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|
{-# LANGUAGE CPP #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE LambdaCase #-}
-- |
-- Module : Unicode.Internal.Unfold
-- Copyright : (c) 2022 Composewell Technologies and Contributors
-- License : BSD-3-Clause
-- Maintainer : streamly@composewell.com
-- Stability : experimental
-- Portability : GHC
--
-- @since 0.3.1
--
module Unicode.Internal.Unfold
( Unfold(..)
, Step(..)
, toList
) where
-- | An @Unfold a b@ is a generator of a stream of values of type @b@ from a
-- seed of type @a@.
--
-- @since 0.3.1
#if MIN_VERSION_base(4,12,0)
data Unfold a b = forall s. Unfold
(s -> Step s b)
-- ^ /Step/ function: compute the next step from the current one.
(a -> Step s b)
-- ^ /Inject/ function: initialize the state with a seed value.
#else
data Unfold a b =
-- | @Unfold step inject@
forall s. Unfold (s -> Step s b) (a -> Step s b)
#endif
-- | A stream is a succession of 'Step's.
--
-- @since 0.3.1
data Step s a
= Yield !a !s
-- ^ Produces a single value and the next state of the stream.
| Stop
-- ^ Indicates there are no more values in the stream.
instance Functor (Step s) where
{-# INLINE fmap #-}
fmap f (Yield x s) = Yield (f x) s
fmap _ Stop = Stop
-- | Convert an 'Unfold a a' to a list [a], if the resulting list is empty the
-- seed is used as a default output.
--
{-# INLINE toList #-}
toList :: Unfold a a -> a -> [a]
toList (Unfold step inject) input =
case inject input of
Stop -> [input]
Yield b s -> b : go (step s)
where
go = \case
Yield b s -> let !s' = step s in b : go s'
Stop -> []
|
