Data.Iteratee.Base

Monadic and General Iteratees: incremental input parsers, processors and transformers

Types

data StreamChunk c el => StreamG c el

A stream is a (continuing) sequence of elements bundled in Chunks. The first two variants indicate termination of the stream. Chunk a gives the currently available part of the stream. The stream is not terminated yet. The case (null Chunk) signifies a stream with no currently available data but which is still continuing. A stream processor should, informally speaking, ``suspend itself'' and wait for more data to arrive.

Constructors

EOF

Error String

Chunk (c el)

data IterateeG s el m a

Iteratee -- a generic stream processor, what is being folded over a stream When Iteratee is in the done state, it contains the computed result and the remaining part of the stream. In the cont state, the iteratee has not finished the computation and needs more input. We assume that all iteratees are good -- given bounded input, they do the bounded amount of computation and take the bounded amount of resources. The monad m describes the sort of computations done by the iteratee as it processes the stream. The monad m could be the identity monad (for pure computations) or the IO monad (to let the iteratee store the stream processing results as they are computed). We also assume that given a terminated stream, an iteratee moves to the done state, so the results computed so far could be returned.

Constructors

Done a (StreamG s el)

Cont (StreamG s el -> IterateeGM s el m a)

Seek FileOffset (StreamG s el -> IterateeGM s el m a)

Instances

Show a => Show (IterateeG s el m a)

newtype IterateeGM s el m a

Constructors

IM

Fields

unIM :: m (IterateeG s el m a)

Instances

StreamChunk s el => MonadTrans (IterateeGM s el)

(StreamChunk s el, Monad m) => Monad (IterateeGM s el m)

(Monad m, Functor m) => Functor (IterateeGM s el m)

(StreamChunk s el, MonadIO m) => MonadIO (IterateeGM s el m)

type EnumeratorN s_outer el_outer s_inner el_inner m a = IterateeG s_inner el_inner m a -> IterateeGM s_outer el_outer m (IterateeG s_inner el_inner m a)

The type of the converter from the stream with elements el_outer to the stream with element el_inner. The result is the iteratee for the outer stream that uses an `IterateeG el_inner m a' to process the embedded, inner stream as it reads the outer stream.

type EnumeratorGM s el m a = IterateeG s el m a -> IterateeGM s el m a

Each enumerator takes an iteratee and returns an iteratee an Enumerator is an iteratee transformer. The enumerator normally stops when the stream is terminated or when the iteratee moves to the done state, whichever comes first. When to stop is of course up to the enumerator...

type EnumeratorGMM sfrom elfrom sto elto m a = IterateeG sto elto m a -> IterateeGM sfrom elfrom m a

More general enumerator type: enumerator that maps streams (not necessarily in lock-step). This is a flattened (`joinI-ed') EnumeratorN sfrom elfrom sto elto m a

Iteratees

Iteratee Combinators

liftI :: Monad m => IterateeG s el m a -> IterateeGM s el m a

Lift an IterateeG into an IterateeGM.

(>>==) :: Monad m => IterateeGM s el m a -> (IterateeG s el m a -> IterateeGM s' el' m b) -> IterateeGM s' el' m b

Just like bind (at run-time, this is indeed exactly bind)

(==<<) :: Monad m => (IterateeG s el m a -> IterateeGM s' el' m b) -> IterateeGM s el m a -> IterateeGM s' el' m b

Just like an application -- a call-by-value-like application

joinI :: (StreamChunk s el, StreamChunk s' el', Monad m) => IterateeGM s el m (IterateeG s' el' m a) -> IterateeGM s el m a

The following is a variant of join in the IterateeGM s el m monad When el' is the same as el, the type of joinI is indeed that of true monadic join. However, joinI is subtly different: since generally el' is different from el, it makes no sense to continue using the internal, IterateeG el' m a: we no longer have elements of the type el' to feed to that iteratee. We thus send EOF to the internal Iteratee and propagate its result. This join function is useful when dealing with `derived iteratees' for embedded/nested streams. In particular, joinI is useful to process the result of take, mapStream, or convStream below.

stream2list :: (StreamChunk s el, Monad m) => IterateeGM s el m [el]

Read a stream to the end and return all of its elements as a list

Error handling

iterErr :: (StreamChunk s el, Monad m) => String -> IterateeGM s el m ()

Report and propagate an error. Disregard the input first and then propagate the error.

iterReportError :: (StreamChunk s el, Monad m) => IterateeGM s el m (Maybe String)

Check to see if the stream is in error

Basic Iteratees

break :: (StreamChunk s el, Monad m) => (el -> Bool) -> IterateeGM s el m (s el, Maybe el)

The analogue of List.break It takes an element predicate and returns a pair: (str, Just c) -- the element c is the first element of the stream satisfying the break predicate; The chunk str is the prefix of the stream up to but including c (str,Nothing) -- The stream is terminated with EOF or error before any element satisfying the break predicate was found. str is the scanned part of the stream. None of the element in str satisfy the break predicate.

dropWhile :: (StreamChunk s el, Monad m) => (el -> Bool) -> IterateeGM s el m ()

A particular optimized case of drop: skip all elements of the stream satisfying the given predicate - until the first element that does not satisfy the predicate, or the end of the stream. This is the analogue of List.dropWhile

drop :: (StreamChunk s el, Monad m) => Int -> IterateeGM s el m ()

Skip n elements of the stream, if there are that many This is the analogue of List.drop

head :: (StreamChunk s el, Monad m) => IterateeGM s el m (Maybe el)

Attempt to read the next element of the stream Return (Just c) if successful, return Nothing if the stream is terminated (by EOF or an error)

peek :: (StreamChunk s el, Monad m) => IterateeGM s el m (Maybe el)

Look ahead at the next element of the stream, without removing it from the stream. Return (Just c) if successful, return Nothing if the stream is terminated (by EOF or an error)

skipToEof :: (StreamChunk s el, Monad m) => IterateeGM s el m ()

Skip the rest of the stream

seek :: (StreamChunk s el, Monad m) => FileOffset -> IterateeGM s el m ()

Create a request to seek within an input stream. This will result in an error if the enumerator is not capable of responding to a seek request.

Advanced iteratee combinators

take :: (StreamChunk s el, Monad m) => Int -> EnumeratorN s el s el m a

Read n elements from a stream and apply the given iteratee to the stream of the read elements. Unless the stream is terminated early, we read exactly n elements (even if the iteratee has accepted fewer).

takeR :: (StreamChunk s el, Monad m) => Int -> EnumeratorN s el s el m a

Read n elements from a stream and apply the given iteratee to the stream of the read elements. If the given iteratee accepted fewer elements, we stop. This is the variation of take with the early termination of processing of the outer stream once the processing of the inner stream finished early. This variation is particularly useful for randomIO, where we do not have to care to `drain the input stream'.

mapStream :: (StreamChunk s el, StreamChunk s el', Monad m) => (el -> el') -> EnumeratorN s el s el' m a

Map the stream: yet another iteratee transformer Given the stream of elements of the type el and the function el->el', build a nested stream of elements of the type el' and apply the given iteratee to it. Note the contravariance

convStream :: (StreamChunk s el, StreamChunk s' el', Monad m) => IterateeGM s el m (Maybe (s' el')) -> EnumeratorN s el s' el' m a

Convert one stream into another, not necessarily in lockstep The transformer mapStream maps one element of the outer stream to one element of the nested stream. The transformer below is more general: it may take several elements of the outer stream to produce one element of the inner stream, or the other way around. The transformation from one stream to the other is specified as IterateeGM s el m (Maybe [el']). The Maybe type reflects the possibility of the conversion error.

Enumerators

enumEof :: (StreamChunk s el, Monad m) => EnumeratorGM s el m a

The most primitive enumerator: applies the iteratee to the terminated stream. The result is the iteratee usually in the done state.

enumErr :: (StreamChunk s el, Monad m) => String -> EnumeratorGM s el m a

Another primitive enumerator: report an error

(>.) :: (StreamChunk s el, Monad m) => EnumeratorGM s el m a -> EnumeratorGM s el m a -> EnumeratorGM s el m a

The composition of two enumerators: essentially the functional composition It is convenient to flip the order of the arguments of the composition though: in e1 >. e2, e1 is executed first

enumPure1Chunk :: (StreamChunk s el, Monad m) => s el -> EnumeratorGM s el m a

The pure 1-chunk enumerator It passes a given list of elements to the iteratee in one chunk This enumerator does no IO and is useful for testing of base parsing

enumPureNChunk :: (StreamChunk s el, Monad m) => s el -> Int -> EnumeratorGM s el m a

The pure n-chunk enumerator It passes a given lift of elements to the iteratee in n chunks This enumerator does no IO and is useful for testing of base parsing and handling of chunk boundaries

Misc.

type FileOffset = COff

bindm :: Monad m => m (Maybe a) -> (a -> m (Maybe b)) -> m (Maybe b)

A useful combinator.