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-----------------------------------------------------------------------------
-- |
-- Module : Data.Stream
-- Copyright : (c) Ross Paterson 2003
-- License : BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer : ross@soi.city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- Infinite sequences.
module Data.Stream where
-- | An infinite sequence.
data Stream a = Cons {
shd :: a, -- ^ head of the stream
stl :: Stream a -- ^ tail of the stream
}
instance Functor Stream where
fmap f xs = Cons (f (shd xs)) (fmap f (stl xs))
instance Monad Stream where
return x = let return_x = Cons x return_x in return_x
xs >>= f = Cons (shd (f (shd xs))) (stl xs >>= f)
-- | Lazy zip of a pair of streams.
zipStream :: Stream a -> Stream b -> Stream (a, b)
zipStream xs ys = Cons (shd xs, shd ys) (zipStream (stl xs) (stl ys))
-- | Lazy unzip of a pair of streams.
unzipStream :: Stream (a, b) -> (Stream a, Stream b)
unzipStream xys = (fmap fst xys, fmap snd xys)
-- | An infinite sequence obtained by padding the list with 'undefined'.
listToStream :: [a] -> Stream a
listToStream = foldr Cons (error "listToStream")
-- | The infinite list corresponding to a stream.
streamToList :: Stream a -> [a]
streamToList xs = shd xs : streamToList (stl xs)
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