Safe Haskell  None 

Language  Haskell2010 
 data Stream f m r
 unfold :: (Monad m, Functor f) => (s > m (Either r (f s))) > s > Stream f m r
 construct :: (forall b. (f b > b) > (m b > b) > (r > b) > b) > Stream f m r
 for :: (Monad m, Functor f) => Stream (Of a) m r > (a > Stream f m x) > Stream f m r
 layer :: (Monad m, Functor f) => f r > Stream f m r
 layers :: (Monad m, Functor f) => Stream (Of a) m r > (a > f x) > Stream f m r
 replicates :: (Monad m, Functor f) => Int > f () > Stream f m ()
 repeats :: (Monad m, Functor f) => f () > Stream f m r
 repeatsM :: (Monad m, Functor f) => m (f ()) > Stream f m r
 delay :: (Monad m, Functor f) => m (Stream f m r) > Stream f m r
 wrap :: (Monad m, Functor f) => f (Stream f m r) > Stream f m r
 decompose :: (Monad m, Functor f) => Stream (Compose m f) m r > Stream f m r
 maps :: (Monad m, Functor f) => (forall x. f x > g x) > Stream f m r > Stream g m r
 mapsM :: (Monad m, Functor f) => (forall x. f x > m (g x)) > Stream f m r > Stream g m r
 distribute :: (Monad m, Functor f, MonadTrans t, MFunctor t, Monad (t (Stream f m))) => Stream f (t m) r > t (Stream f m) r
 eithers :: (Monad m, Applicative h) => (forall x. f x > h x) > (forall x. g x > h x) > Stream (Sum f g) m r > Stream h m r
 inspect :: (Functor f, Monad m) => Stream f m r > m (Either r (f (Stream f m r)))
 zips :: (Monad m, Functor f, Functor g) => Stream f m r > Stream g m r > Stream (Compose f g) m r
 zipsWith :: (Monad m, Functor h) => (forall x y. f x > g y > h (x, y)) > Stream f m r > Stream g m r > Stream h m r
 interleaves :: (Monad m, Applicative h) => Stream h m r > Stream h m r > Stream h m r
 iterTM :: (Functor f, Monad m, MonadTrans t, Monad (t m)) => (f (t m a) > t m a) > Stream f m a > t m a
 iterT :: (Functor f, Monad m) => (f (m a) > m a) > Stream f m a > m a
 destroy :: (Functor f, Monad m) => Stream f m r > (f b > b) > (m b > b) > (r > b) > b
 mapsM_ :: (Functor f, Monad m) => (forall x. f x > m x) > Stream f m r > m r
 run :: Monad m => Stream m m r > m r
 splitsAt :: (Monad m, Functor f) => Int > Stream f m r > Stream f m (Stream f m r)
 takes :: (Monad m, Functor f) => Int > Stream f m r > Stream f m ()
 chunksOf :: (Monad m, Functor f) => Int > Stream f m r > Stream (Stream f m) m r
 concats :: (Monad m, Functor f) => Stream (Stream f m) m r > Stream f m r
 intercalates :: (Monad m, Monad (t m), MonadTrans t) => t m a > Stream (t m) m b > t m b
 data Of a b = !a :> b
 lazily :: Of a b > (a, b)
 strictly :: (a, b) > Of a b
 class MFunctor t where
 class (MFunctor t, MonadTrans t) => MMonad t where
 class MonadTrans t where
 class Monad m => MonadIO m where
 newtype Compose f g a :: (* > *) > (* > *) > * > * = Compose {
 getCompose :: f (g a)
 join :: Monad m => m (m a) > m a
 liftA2 :: Applicative f => (a > b > c) > f a > f b > f c
 liftA3 :: Applicative f => (a > b > c > d) > f a > f b > f c > f d
 void :: Functor f => f a > f ()
Free monad transformer
The Stream
data type is equivalent to FreeT
and can represent any effectful
succession of steps, where the form of the steps or commands
is
specified by the first (functor) parameter. The (hidden) implementation is
data Stream f m r = Step !(f (Stream f m r))  Delay (m (Stream f m r))  Return r
In the simplest case, the base functor is (,) a
. Here the news
or command at each step is an individual element of type a
,
i.e. the command is a yield
statement. The associated
Streaming
Prelude
uses the leftstrict pair Of a b
in place of the Haskell pair (a,b)
In it, various operations are defined for fundamental streaming types like
Stream (Of a) m r  a generator or producer (in the pipes sense)  compare [a], or rather ([a],r) Stream (Of a) m (Stream (Of a) m r)  the effectful splitting of a producer  compare ([a],[a]) or rather ([a],([a],r)) Stream (Stream (Of a) m) m r  segmentation of a producer  cp. [[a]], or rather ([a],([a],([a],(...,r))))
and so on. But of course any functor can be used, and this is part of
the point of this prelude  as we already see from
the type of the segmented stream, Stream (Stream (Of a) m) m r
and operations like e.g.
chunksOf :: Monad m => Int > Stream f m r > Stream (Stream f m) m r mapsM Streaming.Prelude.length' :: Stream (Stream (Of a) m) r > Stream (Of Int) m r
To avoid breaking reasoning principles, the constructors
should not be used directly. A patternmatch should go by way of inspect
 or, in the producer case, next
. These mirror
the type of runFreeT
. The constructors are exported by the Internal
module.
Functor f => MFunctor (Stream f) Source  
Functor f => MMonad (Stream f) Source  
Functor f => MonadTrans (Stream f) Source  
(Functor f, Monad m) => Monad (Stream f m) Source  
(Functor f, Monad m) => Functor (Stream f m) Source  
(Functor f, Monad m) => Applicative (Stream f m) Source  
(MonadIO m, Functor f) => MonadIO (Stream f m) Source  
(Eq r, Eq (m (Stream f m r)), Eq (f (Stream f m r))) => Eq (Stream f m r) Source  
(Typeable (* > *) f, Typeable (* > *) m, Data r, Data (m (Stream f m r)), Data (f (Stream f m r))) => Data (Stream f m r) Source  
(Show r, Show (m (Stream f m r)), Show (f (Stream f m r))) => Show (Stream f m r) Source 
Constructing a Stream
on a base functor
unfold :: (Monad m, Functor f) => (s > m (Either r (f s))) > s > Stream f m r Source
Build a Stream
by unfolding steps starting from a seed. See also
the specialized unfoldr
in the prelude.
unfold inspect = id  modulo the quotient we work with unfold Pipes.next :: Monad m => Producer a m r > Stream ((,) a) m r unfold (curry (:>) . Pipes.next) :: Monad m => Producer a m r > Stream (Of a) m r
construct :: (forall b. (f b > b) > (m b > b) > (r > b) > b) > Stream f m r Source
Reflect a churchencoded stream; cp. GHC.Exts.build
for :: (Monad m, Functor f) => Stream (Of a) m r > (a > Stream f m x) > Stream f m r Source
for
replaces each element of a stream with an associated stream. Note that the
associated stream may layer any functor.
layer :: (Monad m, Functor f) => f r > Stream f m r Source
Lift for items in the base functor. Makes a singleton or onelayer succession.`
replicates :: (Monad m, Functor f) => Int > f () > Stream f m () Source
Repeat a functorial layer, command or instruct several times.
repeats :: (Monad m, Functor f) => f () > Stream f m r Source
Repeat a functorial layer, command or instruction forever.
Transforming streams
decompose :: (Monad m, Functor f) => Stream (Compose m f) m r > Stream f m r Source
Resort a succession of layers of the form m (f x)
. Though mapsM
is best understood as:
mapsM phi = decompose . maps (Compose . phi)
we could as well define decompose
by mapsM
:
decompose = mapsM getCompose
maps :: (Monad m, Functor f) => (forall x. f x > g x) > Stream f m r > Stream g m r Source
Map layers of one functor to another with a transformation
mapsM :: (Monad m, Functor f) => (forall x. f x > m (g x)) > Stream f m r > Stream g m r Source
Map layers of one functor to another with a transformation involving the base monad
maps
is more fundamental than mapsM
, which is best understood as a convenience
for effecting this frequent composition:
mapsM phi = decompose . maps (Compose . phi)
distribute :: (Monad m, Functor f, MonadTrans t, MFunctor t, Monad (t (Stream f m))) => Stream f (t m) r > t (Stream f m) r Source
Make it possible to 'run' the underlying transformed monad.
eithers :: (Monad m, Applicative h) => (forall x. f x > h x) > (forall x. g x > h x) > Stream (Sum f g) m r > Stream h m r Source
Inspecting a stream
inspect :: (Functor f, Monad m) => Stream f m r > m (Either r (f (Stream f m r))) Source
Inspect the first stage of a freely layered sequence.
Compare Pipes.next
and the replica Streaming.Prelude.next
.
This is the uncons
for the general unfold
.
unfold inspect = id Streaming.Prelude.unfoldr StreamingPrelude.next = id
Zipping streams
zips :: (Monad m, Functor f, Functor g) => Stream f m r > Stream g m r > Stream (Compose f g) m r Source
zipsWith :: (Monad m, Functor h) => (forall x y. f x > g y > h (x, y)) > Stream f m r > Stream g m r > Stream h m r Source
interleaves :: (Monad m, Applicative h) => Stream h m r > Stream h m r > Stream h m r Source
Interleave functor layers, with the effects of the first preceding the effects of the second.
interleaves = zipsWith (liftA2 (,))
>>>
let paste = \a b > interleaves (Q.lines a) (maps (Q.cons' '\t') (Q.lines b))
>>>
Q.stdout $ Q.unlines $ paste "hello\nworld\n" "goodbye\nworld\n"
hello goodbye world world
Eliminating a Stream
iterTM :: (Functor f, Monad m, MonadTrans t, Monad (t m)) => (f (t m a) > t m a) > Stream f m a > t m a Source
Specialized fold
iterTM alg stream = destroy stream alg (join . lift) return
iterT :: (Functor f, Monad m) => (f (m a) > m a) > Stream f m a > m a Source
Specialized fold
iterT alg stream = destroy stream alg join return
destroy :: (Functor f, Monad m) => Stream f m r > (f b > b) > (m b > b) > (r > b) > b Source
Map a stream directly to its church encoding; compare Data.List.foldr
It permits distinctions that should be hidden, as can be seen from
e.g.
isPure stream = destroy_ (const True) (const False) (const True)
and similar nonsense. The crucial
constraint is that the m x > x
argument is an EilenbergMoore algebra.
See Atkey "Reasoning about Stream Processing with Effects"
The destroy exported by the safe modules is
destroy str = destroy (observe str)
mapsM_ :: (Functor f, Monad m) => (forall x. f x > m x) > Stream f m r > m r Source
Map each layer to an effect in the base monad, and run them all.
Splitting and joining Stream
s
splitsAt :: (Monad m, Functor f) => Int > Stream f m r > Stream f m (Stream f m r) Source
Split a succession of layers after some number, returning a streaming or effectful pair.
>>>
rest < S.print $ S.splitAt 1 $ each [1..3]
1>>>
S.print rest
2 3
chunksOf :: (Monad m, Functor f) => Int > Stream f m r > Stream (Stream f m) m r Source
Break a stream into substreams each with n functorial layers.
>>>
S.print $ maps' sum' $ chunksOf 2 $ each [1,1,1,1,1,1,1]
2 2 2 1
concats :: (Monad m, Functor f) => Stream (Stream f m) m r > Stream f m r Source
Dissolves the segmentation into layers of Stream f m
layers.
concats stream = destroy stream join (join . lift) return
>>>
S.print $ concats $ maps (cons 1776) $ chunksOf 2 (each [1..5])
1776 1 2 1776 3 4 1776 5
intercalates :: (Monad m, Monad (t m), MonadTrans t) => t m a > Stream (t m) m b > t m b Source
Interpolate a layer at each segment. This specializes to e.g.
intercalates :: (Monad m, Functor f) => Stream f m () > Stream (Stream f m) m r > Stream f m r
Base functor for streams of individual items
A leftstrict pair; the base functor for streams of individual elements.
!a :> b infixr 5 
Monoid a => Monad (Of a) Source  
Functor (Of a) Source  
Monoid a => Applicative (Of a) Source  
Foldable (Of a) Source  
Traversable (Of a) Source  
(Eq a, Eq b) => Eq (Of a b) Source  
(Data a, Data b) => Data (Of a b) Source  
(Ord a, Ord b) => Ord (Of a b) Source  
(Read a, Read b) => Read (Of a b) Source  
(Show a, Show b) => Show (Of a b) Source  
(Monoid a, Monoid b) => Monoid (Of a b) Source 
reexports
class MFunctor t where
A functor in the category of monads, using hoist
as the analog of fmap
:
hoist (f . g) = hoist f . hoist g hoist id = id
hoist :: Monad m => (forall a. m a > n a) > t m b > t n b
Lift a monad morphism from m
to n
into a monad morphism from
(t m)
to (t n)
MFunctor ListT  
MFunctor Backwards  
MFunctor MaybeT  
MFunctor IdentityT  
MFunctor Lift  
MFunctor (ReaderT r)  
MFunctor (StateT s)  
MFunctor (StateT s)  
MFunctor (ExceptT e)  
MFunctor (ErrorT e)  
MFunctor (WriterT w)  
MFunctor (WriterT w)  
MFunctor (Product f)  
Functor f => MFunctor (Compose f)  
Functor f => MFunctor (Stream f)  
MFunctor (RWST r w s)  
MFunctor (RWST r w s) 
class (MFunctor t, MonadTrans t) => MMonad t where
A monad in the category of monads, using lift
from MonadTrans
as the
analog of return
and embed
as the analog of (=<<
):
embed lift = id embed f (lift m) = f m embed g (embed f t) = embed (\m > embed g (f m)) t
class MonadTrans t where
The class of monad transformers. Instances should satisfy the
following laws, which state that lift
is a monad transformation:
MonadTrans ListT  
MonadTrans MaybeT  
MonadTrans IdentityT  
MonadTrans (ReaderT r)  
MonadTrans (StateT s)  
MonadTrans (StateT s)  
MonadTrans (ExceptT e)  
Error e => MonadTrans (ErrorT e)  
Monoid w => MonadTrans (WriterT w)  
Monoid w => MonadTrans (WriterT w)  
Functor f => MonadTrans (Stream f)  
Monoid w => MonadTrans (RWST r w s)  
Monoid w => MonadTrans (RWST r w s) 
class Monad m => MonadIO m where
Monads in which IO
computations may be embedded.
Any monad built by applying a sequence of monad transformers to the
IO
monad will be an instance of this class.
Instances should satisfy the following laws, which state that liftIO
is a transformer of monads:
MonadIO IO  
MonadIO m => MonadIO (ListT m)  
MonadIO m => MonadIO (MaybeT m)  
MonadIO m => MonadIO (IdentityT m)  
MonadIO m => MonadIO (ReaderT r m)  
MonadIO m => MonadIO (StateT s m)  
MonadIO m => MonadIO (StateT s m)  
MonadIO m => MonadIO (ExceptT e m)  
(Error e, MonadIO m) => MonadIO (ErrorT e m)  
(Monoid w, MonadIO m) => MonadIO (WriterT w m)  
(Monoid w, MonadIO m) => MonadIO (WriterT w m)  
(MonadIO m, Functor f) => MonadIO (Stream f m)  
(Monoid w, MonadIO m) => MonadIO (RWST r w s m)  
(Monoid w, MonadIO m) => MonadIO (RWST r w s m) 
newtype Compose f g a :: (* > *) > (* > *) > * > * infixr 9
Righttoleft composition of functors. The composition of applicative functors is always applicative, but the composition of monads is not always a monad.
Compose infixr 9  

Functor f => MFunctor (Compose f)  
(Functor f, Functor g) => Functor (Compose f g)  
(Applicative f, Applicative g) => Applicative (Compose f g)  
(Foldable f, Foldable g) => Foldable (Compose f g)  
(Traversable f, Traversable g) => Traversable (Compose f g)  
(Alternative f, Applicative g) => Alternative (Compose f g)  
(Functor f, Eq1 f, Eq1 g) => Eq1 (Compose f g)  
(Functor f, Ord1 f, Ord1 g) => Ord1 (Compose f g)  
(Functor f, Read1 f, Read1 g) => Read1 (Compose f g)  
(Functor f, Show1 f, Show1 g) => Show1 (Compose f g)  
(Functor f, Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a)  
(Functor f, Ord1 f, Ord1 g, Ord a) => Ord (Compose f g a)  
(Functor f, Read1 f, Read1 g, Read a) => Read (Compose f g a)  
(Functor f, Show1 f, Show1 g, Show a) => Show (Compose f g a) 
join :: Monad m => m (m a) > m a
The join
function is the conventional monad join operator. It
is used to remove one level of monadic structure, projecting its
bound argument into the outer level.
liftA2 :: Applicative f => (a > b > c) > f a > f b > f c
Lift a binary function to actions.
liftA3 :: Applicative f => (a > b > c > d) > f a > f b > f c > f d
Lift a ternary function to actions.
void :: Functor f => f a > f ()
discards or ignores the result of evaluation, such
as the return value of an void
valueIO
action.
Examples
Replace the contents of a
with unit:Maybe
Int
>>>
void Nothing
Nothing>>>
void (Just 3)
Just ()
Replace the contents of an
with unit,
resulting in an Either
Int
Int
:Either
Int
'()'
>>>
void (Left 8675309)
Left 8675309>>>
void (Right 8675309)
Right ()
Replace every element of a list with unit:
>>>
void [1,2,3]
[(),(),()]
Replace the second element of a pair with unit:
>>>
void (1,2)
(1,())
Discard the result of an IO
action:
>>>
mapM print [1,2]
1 2 [(),()]>>>
void $ mapM print [1,2]
1 2