Safe Haskell | Trustworthy |
---|---|

Language | Haskell2010 |

- fromLazy :: Monad m => Text -> Producer' Text m ()
- map :: Monad m => (Char -> Char) -> Pipe Text Text m r
- concatMap :: Monad m => (Char -> Text) -> Pipe Text Text m r
- take :: (Monad m, Integral a) => a -> Pipe Text Text m ()
- drop :: (Monad m, Integral a) => a -> Pipe Text Text m r
- takeWhile :: Monad m => (Char -> Bool) -> Pipe Text Text m ()
- dropWhile :: Monad m => (Char -> Bool) -> Pipe Text Text m r
- filter :: Monad m => (Char -> Bool) -> Pipe Text Text m r
- scan :: Monad m => (Char -> Char -> Char) -> Char -> Pipe Text Text m r
- pack :: Monad m => Pipe String Text m r
- unpack :: Monad m => Pipe Text String m r
- toCaseFold :: Monad m => Pipe Text Text m r
- toLower :: Monad m => Pipe Text Text m r
- toUpper :: Monad m => Pipe Text Text m r
- stripStart :: Monad m => Pipe Text Text m r
- toLazy :: Producer Text Identity () -> Text
- toLazyM :: Monad m => Producer Text m () -> m Text
- foldChars :: Monad m => (x -> Char -> x) -> x -> (x -> r) -> Producer Text m () -> m r
- head :: Monad m => Producer Text m () -> m (Maybe Char)
- last :: Monad m => Producer Text m () -> m (Maybe Char)
- null :: Monad m => Producer Text m () -> m Bool
- length :: (Monad m, Num n) => Producer Text m () -> m n
- any :: Monad m => (Char -> Bool) -> Producer Text m () -> m Bool
- all :: Monad m => (Char -> Bool) -> Producer Text m () -> m Bool
- maximum :: Monad m => Producer Text m () -> m (Maybe Char)
- minimum :: Monad m => Producer Text m () -> m (Maybe Char)
- find :: Monad m => (Char -> Bool) -> Producer Text m () -> m (Maybe Char)
- index :: (Monad m, Integral a) => a -> Producer Text m () -> m (Maybe Char)
- count :: (Monad m, Num n) => Text -> Producer Text m () -> m n
- nextChar :: Monad m => Producer Text m r -> m (Either r (Char, Producer Text m r))
- drawChar :: Monad m => Parser Text m (Maybe Char)
- unDrawChar :: Monad m => Char -> Parser Text m ()
- peekChar :: Monad m => Parser Text m (Maybe Char)
- isEndOfChars :: Monad m => Parser Text m Bool
- splitAt :: (Monad m, Integral n) => n -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- span :: Monad m => (Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- break :: Monad m => (Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- groupBy :: Monad m => (Char -> Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- group :: Monad m => Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- word :: Monad m => Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- line :: Monad m => Lens' (Producer Text m r) (Producer Text m (Producer Text m r))
- chunksOf :: (Monad m, Integral n) => n -> Lens' (Producer Text m r) (FreeT (Producer Text m) m r)
- splitsWith :: Monad m => (Char -> Bool) -> Producer Text m r -> FreeT (Producer Text m) m r
- splits :: Monad m => Char -> Lens' (Producer Text m r) (FreeT (Producer Text m) m r)
- groupsBy :: Monad m => (Char -> Char -> Bool) -> Lens' (Producer Text m x) (FreeT (Producer Text m) m x)
- groups :: Monad m => Lens' (Producer Text m x) (FreeT (Producer Text m) m x)
- lines :: Monad m => Iso' (Producer Text m r) (FreeT (Producer Text m) m r)
- words :: Monad m => Iso' (Producer Text m r) (FreeT (Producer Text m) m r)
- intersperse :: Monad m => Char -> Producer Text m r -> Producer Text m r
- packChars :: Monad m => Iso' (Producer Char m x) (Producer Text m x)
- intercalate :: Monad m => Producer Text m () -> FreeT (Producer Text m) m r -> Producer Text m r
- unlines :: Monad m => FreeT (Producer Text m) m r -> Producer Text m r
- unwords :: Monad m => FreeT (Producer Text m) m r -> Producer Text m r
- module Data.ByteString
- module Data.Text
- module Data.Profunctor
- module Pipes.Parse
- module Pipes.Group

# Effectful Text

This package provides `pipes`

utilities for *text streams* or *character streams*,
realized as streams of `Text`

chunks. The individual chunks are uniformly *strict*,
and thus you will generally want `Data.Text`

in scope. But the type
`Producer Text m r`

,as we are using it, is a sort of *pipes* equivalent of the lazy `Text`

type.

This particular module provides many functions equivalent in one way or another to
the pure functions in
Data.Text.Lazy.
They transform, divide, group and fold text streams. Though `Producer Text m r`

is the type of 'effectful Text', the functions in this module are 'pure'
in the sense that they are uniformly monad-independent.
Simple *IO* operations are defined in `Pipes.Text.IO`

-- as lazy IO `Text`

operations are in `Data.Text.Lazy.IO`

. Inter-operation with `ByteString`

is provided in `Pipes.Text.Encoding`

, which parallels `Data.Text.Lazy.Encoding`

.

The Text type exported by `Data.Text.Lazy`

is basically that of a lazy list of
strict Text: the implementation is arranged so that the individual strict `Text`

chunks are kept to a reasonable size; the user is not aware of the divisions
between the connected `Text`

chunks.
So also here: the functions in this module are designed to operate on streams that
are insensitive to text boundaries. This means that they may freely split
text into smaller texts and *discard empty texts*. The objective, though, is
that they should *never concatenate texts* in order to provide strict upper
bounds on memory usage.

For example, to stream only the first three lines of `stdin`

to `stdout`

you
might write:

import Pipes import qualified Pipes.Text as Text import qualified Pipes.Text.IO as Text import Pipes.Group (takes') import Lens.Family main = runEffect $ takeLines 3 Text.stdin >-> Text.stdout where takeLines n = Text.unlines . takes' n . view Text.lines

The above program will never bring more than one chunk of text (~ 32 KB) into memory, no matter how long the lines are.

# Lenses

As this example shows, one superficial difference from `Data.Text.Lazy`

is that many of the operations, like `lines`

, are 'lensified'; this has a
number of advantages (where it is possible); in particular it facilitates their
use with `Parser`

s of Text (in the general pipes-parse
sense.) The disadvantage, famously, is that the messages you get for type errors can be
a little alarming. The remarks that follow in this section are for non-lens adepts.

Each lens exported here, e.g. `lines`

, `chunksOf`

or `splitAt`

, reduces to the
intuitively corresponding function when used with `view`

or `(^.)`

. Instead of
writing:

splitAt 17 producer

as we would with the Prelude or Text functions, we write

view (splitAt 17) producer

or equivalently

producer ^. splitAt 17

This may seem a little indirect, but note that many equivalents of
`Text -> Text`

functions are exported here as `Pipe`

s. Here too we recover the intuitively
corresponding functions by prefixing them with `(>->)`

. Thus something like

stripLines = Text.unlines . Group.maps (>-> Text.stripStart) . view Text.lines

would drop the leading white space from each line.

The lenses in this library are marked as *improper*; this just means that
they don't admit all the operations of an ideal lens, but only *getting* and *focusing*.
Just for this reason, though, the magnificent complexities of the lens libraries
are a distraction. The lens combinators to keep in mind, the ones that make sense for
our lenses, are `view`

/ `(^.)`

), `over`

/ `(%~)`

, and `zoom`

.

One need only keep in mind that if `l`

is a `Lens' a b`

, then:

`view`

/ `(^.)`

`view l`

is a function `a -> b`

. Thus `view l a`

(also written `a ^. l`

)
is the corresponding `b`

; as was said above, this function will be exactly the
function you think it is, given its name. Thus to uppercase the first n characters
of a Producer, leaving the rest the same, we could write:

upper n p = do p' <- p ^. Text.splitAt n >-> Text.toUpper p'

`over`

/ `(%~)`

`over l`

is a function `(b -> b) -> a -> a`

. Thus, given a function that modifies
`b`

s, the lens lets us modify an `a`

by applying `f :: b -> b`

to
the `b`

that we can "see" through the lens. So `over l f :: a -> a`

(it can also be written `l %~ f`

).
For any particular `a`

, then, `over l f a`

or `(l %~ f) a`

is a revised `a`

.
So above we might have written things like these:

stripLines = Text.lines %~ maps (>-> Text.stripStart) stripLines = over Text.lines (maps (>-> Text.stripStart)) upper n = Text.splitAt n %~ (>-> Text.toUpper)

`zoom`

`zoom l`

, finally, is a function from a `Parser b m r`

to a `Parser a m r`

(or more generally a `StateT (Producer b m x) m r`

).
Its use is easiest to see with an decoding lens like `utf8`

, which
"sees" a Text producer hidden inside a ByteString producer:
`drawChar`

is a Text parser, returning a `Maybe Char`

, `zoom utf8 drawChar`

is
a *ByteString* parser, returning a `Maybe Char`

. `drawAll`

is a Parser that returns
a list of everything produced from a Producer, leaving only the return value; it would
usually be unreasonable to use it. But `zoom (splitAt 17) drawAll`

returns a list of Text chunks containing the first seventeen Chars, and returns the rest of
the Text Producer for further parsing. Suppose that we want, inexplicably, to
modify the casing of a Text Producer according to any instruction it might
contain at the start. Then we might write something like this:

obey :: Monad m => Producer Text m b -> Producer Text m b obey p = do (ts, p') <- lift $ runStateT (zoom (Text.splitAt 7) drawAll) p let seven = T.concat ts case T.toUpper seven of "TOUPPER" -> p' >-> Text.toUpper "TOLOWER" -> p' >-> Text.toLower _ -> do yield seven p'

>>> let doc = each ["toU","pperTh","is document.\n"] >>> runEffect $ obey doc >-> Text.stdout THIS DOCUMENT.

The purpose of exporting lenses is the mental economy achieved with this three-way
applicability. That one expression, e.g. `lines`

or `splitAt 17`

can have these
three uses is no more surprising than that a pipe can act as a function modifying
the output of a producer, namely by using `>->`

to its left: `producer >-> pipe`

-- but can *also* modify the inputs to a consumer by using `>->`

to its right:
`pipe >-> consumer`

The three functions, `view`

/ `(^.)`

, `over`

/ `(%~)`

and `zoom`

are supplied by
both lens and
lens-family The use of `zoom`

is explained
in Pipes.Parse.Tutorial
and to some extent in the `Pipes.Text.Encoding`

module here.

# Special types: `Producer Text m (Producer Text m r)`

and `FreeT (Producer Text m) m r`

These simple `lines`

examples reveal a more important difference from `Data.Text.Lazy`

.
This is in the types that are most closely associated with our central text type,
`Producer Text m r`

. In `Data.Text`

and `Data.Text.Lazy`

we find functions like

splitAt :: Int -> Text -> (Text, Text) lines :: Text -> [Text] chunksOf :: Int -> Text -> [Text]

which relate a Text with a pair of Texts or a list of Texts. The corresponding functions here (taking account of 'lensification') are

view . splitAt :: (Monad m, Integral n) => n -> Producer Text m r -> Producer Text m (Producer Text m r) view lines :: Monad m => Producer Text m r -> FreeT (Producer Text m) m r view . chunksOf :: (Monad m, Integral n) => n -> Producer Text m r -> FreeT (Producer Text m) m r

Some of the types may be more readable if you imagine that we have introduced our own type synonyms

type Text m r = Producer T.Text m r type Texts m r = FreeT (Producer T.Text m) m r

Then we would think of the types above as

view . splitAt :: (Monad m, Integral n) => n -> Text m r -> Text m (Text m r) view lines :: (Monad m) => Text m r -> Texts m r view . chunksOf :: (Monad m, Integral n) => n -> Text m r -> Texts m r

which brings one closer to the types of the similar functions in `Data.Text.Lazy`

In the type `Producer Text m (Producer Text m r)`

the second
element of the 'pair' of effectful Texts cannot simply be retrieved
with something like `snd`

. This is an 'effectful' pair, and one must work
through the effects of the first element to arrive at the second Text stream, even
if you are proposing to throw the Text in the first element away.
Note that we use Control.Monad.join to fuse the pair back together, since it specializes to

join :: Monad m => Producer Text m (Producer m r) -> Producer m r

The return type of `lines`

, `words`

, `chunksOf`

and the other *splitter* functions,
`FreeT (Producer m Text) m r`

-- our `Texts m r`

-- is the type of (effectful)
lists of (effectful) texts. The type `([Text],r)`

might be seen to gather
together things of the forms:

r (Text,r) (Text, (Text, r)) (Text, (Text, (Text, r))) (Text, (Text, (Text, (Text, r)))) ...

(We might also have identified the sum of those types with `Free ((,) Text) r`

-- or, more absurdly, `FreeT ((,) Text) Identity r`

.)

Similarly, our type `Texts m r`

, or `FreeT (Text m) m r`

-- in fact called
`FreeT (Producer Text m) m r`

here -- encompasses all the members of the sequence:

m r Text m r Text m (Text m r) Text m (Text m (Text m r)) Text m (Text m (Text m (Text m r))) ...

We might have used a more specialized type in place of `FreeT (Producer a m) m r`

,
or indeed of `FreeT (Producer Text m) m r`

, but it is clear that the correct
result type of `lines`

will be isomorphic to `FreeT (Producer Text m) m r`

.

One might think that

lines :: Monad m => Lens' (Producer Text m r) (FreeT (Producer Text m) m r) view . lines :: Monad m => Producer Text m r -> FreeT (Producer Text m) m r

should really have the type

lines :: Monad m => Pipe Text Text m r

as e.g. `toUpper`

does. But this would spoil the control we are
attempting to maintain over the size of chunks. It is in fact just
as unreasonable to want such a pipe as to want

Data.Text.Lazy.lines :: Text -> Text

to `rechunk`

the strict Text chunks inside the lazy Text to respect
line boundaries. In fact we have

Data.Text.Lazy.lines :: Text -> [Text] Prelude.lines :: String -> [String]

where the elements of the list are themselves lazy Texts or Strings; the use
of `FreeT (Producer Text m) m r`

is simply the `effectful`

version of this.

The `Pipes.Group`

module, which can generally be imported without qualification,
provides many functions for working with things of type `FreeT (Producer a m) m r`

.
In particular it conveniently exports the constructors for `FreeT`

and the associated
`FreeF`

type -- a fancy form of `Either`

, namely

data FreeF f a b = Pure a | Free (f b)

for pattern-matching. Consider the implementation of the `words`

function, or
of the part of the lens that takes us to the words; it is compact but exhibits many
of the points under discussion, including explicit handling of the `FreeT`

and `FreeF`

constuctors. Keep in mind that

newtype FreeT f m a = FreeT (m (FreeF f a (FreeT f m a))) next :: Monad m => Producer a m r -> m (Either r (a, Producer a m r))

Thus the `do`

block after the `FreeT`

constructor is in the base monad, e.g. `IO`

or `Identity`

;
the later subordinate block, opened by the `Free`

constructor, is in the `Producer`

monad:

words :: Monad m => Producer Text m r -> FreeT (Producer Text m) m r words p = FreeT $ do -- With 'next' we will inspect p's first chunk, excluding spaces; x <- next (p >-> dropWhile isSpace) -- note that 'dropWhile isSpace' is a pipe, and is thus *applied* with '>->'. return $ case x of -- We use 'return' and so need something of type 'FreeF (Text m) r (Texts m r)' Left r -> Pure r -- 'Left' means we got no Text chunk, but only the return value; so we are done. Right (txt, p') -> Free $ do -- If we get a chunk and the rest of the producer, p', we enter the 'Producer' monad p'' <- view (break isSpace) -- When we apply 'break isSpace', we get a Producer that returns a Producer; (yield txt >> p') -- so here we yield everything up to the next space, and get the rest back. return (words p'') -- We then carry on with the rest, which is likely to begin with space.

# Producers

# Pipes

map :: Monad m => (Char -> Char) -> Pipe Text Text m r

Apply a transformation to each `Char`

in the stream

concatMap :: Monad m => (Char -> Text) -> Pipe Text Text m r

Map a function over the characters of a text stream and concatenate the results

take :: (Monad m, Integral a) => a -> Pipe Text Text m ()

`(take n)`

only allows `n`

individual characters to pass;
contrast `Pipes.Prelude.take`

which would let `n`

chunks pass.

takeWhile :: Monad m => (Char -> Bool) -> Pipe Text Text m ()

Take characters until they fail the predicate

dropWhile :: Monad m => (Char -> Bool) -> Pipe Text Text m r

Drop characters until they fail the predicate

filter :: Monad m => (Char -> Bool) -> Pipe Text Text m r

Only allows `Char`

s to pass if they satisfy the predicate

scan :: Monad m => (Char -> Char -> Char) -> Char -> Pipe Text Text m r

Strict left scan over the characters

toCaseFold :: Monad m => Pipe Text Text m r

stripStart :: Monad m => Pipe Text Text m r

Remove leading white space from an incoming succession of `Text`

s

# Folds

foldChars :: Monad m => (x -> Char -> x) -> x -> (x -> r) -> Producer Text m () -> m r

Reduce the text stream using a strict left fold over characters

maximum :: Monad m => Producer Text m () -> m (Maybe Char)

Return the maximum `Char`

within a text stream

minimum :: Monad m => Producer Text m () -> m (Maybe Char)

Return the minimum `Char`

within a text stream (surely very useful!)

find :: Monad m => (Char -> Bool) -> Producer Text m () -> m (Maybe Char)

Find the first element in the stream that matches the predicate

count :: (Monad m, Num n) => Text -> Producer Text m () -> m n

Store a tally of how many segments match the given `Text`

# Primitive Character Parsers

peekChar :: Monad m => Parser Text m (Maybe Char)

`peekChar`

checks the first `Char`

in the stream, but uses `unDrawChar`

to
push the `Char`

back

peekChar = do x <- drawChar case x of Left _ -> return () Right c -> unDrawChar c return x

isEndOfChars :: Monad m => Parser Text m Bool

Check if the underlying `Producer`

has no more characters

Note that this will skip over empty `Text`

chunks, unlike
`isEndOfInput`

from `pipes-parse`

, which would consider
an empty `Text`

a valid bit of input.

isEndOfChars = liftM isLeft peekChar

# Parsing Lenses

splitAt :: (Monad m, Integral n) => n -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))

Splits a `Producer`

after the given number of characters

span :: Monad m => (Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))

Split a text stream in two, producing the longest consecutive group of characters that satisfies the predicate and returning the rest

break :: Monad m => (Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))

Split a text stream in two, producing the longest consecutive group of characters that don't satisfy the predicate

groupBy :: Monad m => (Char -> Char -> Bool) -> Lens' (Producer Text m r) (Producer Text m (Producer Text m r))

Improper lens that splits after the first group of equivalent Chars, as defined by the given equivalence relation

group :: Monad m => Lens' (Producer Text m r) (Producer Text m (Producer Text m r))

Improper lens that splits after the first succession of identical `Char`

s

# FreeT Splitters

chunksOf :: (Monad m, Integral n) => n -> Lens' (Producer Text m r) (FreeT (Producer Text m) m r)

Split a text stream into `FreeT`

-delimited text streams of fixed size

splitsWith :: Monad m => (Char -> Bool) -> Producer Text m r -> FreeT (Producer Text m) m r

Split a text stream into sub-streams delimited by characters that satisfy the predicate

splits :: Monad m => Char -> Lens' (Producer Text m r) (FreeT (Producer Text m) m r)

Split a text stream using the given `Char`

as the delimiter

groupsBy :: Monad m => (Char -> Char -> Bool) -> Lens' (Producer Text m x) (FreeT (Producer Text m) m x)

lines :: Monad m => Iso' (Producer Text m r) (FreeT (Producer Text m) m r)

Split a text stream into `FreeT`

-delimited lines

words :: Monad m => Iso' (Producer Text m r) (FreeT (Producer Text m) m r)

Split a text stream into `FreeT`

-delimited words

# Transformations

packChars :: Monad m => Iso' (Producer Char m x) (Producer Text m x)

Improper isomorphism between a `Producer`

of `ByteString`

s and `Word8`

s

# Joiners

intercalate :: Monad m => Producer Text m () -> FreeT (Producer Text m) m r -> Producer Text m r

`intercalate`

concatenates the `FreeT`

-delimited text streams after
interspersing a text stream in between them

unlines :: Monad m => FreeT (Producer Text m) m r -> Producer Text m r

Join `FreeT`

-delimited lines into a text stream

unwords :: Monad m => FreeT (Producer Text m) m r -> Producer Text m r

Join `FreeT`

-delimited words into a text stream

# Re-exports

`Data.Text`

re-exports the `Text`

type.

`Pipes.Parse`

re-exports `input`

, `concat`

, `FreeT`

(the type) and the `Parse`

synonym.

module Data.ByteString

module Data.Text

module Data.Profunctor

module Pipes.Parse

module Pipes.Group