Pipes.Text.Tutorial

Effectful Text

This package provides pipes utilities for character streams, realized as streams of Text chunks. The individual chunks are uniformly strict, and thus the Text type we are using is the one from Data.Text, not Data.Text.Lazy But the type Producer Text m r, as we are using it, is a sort of pipes equivalent of the lazy Text type.

Pipes.Text

The main Pipes.Text module provides many functions equivalent in one way or another to the pure functions in Data.Text.Lazy (and the corresponding Prelude functions for String s): they transform, divide, group and fold text streams. Though Producer Text m r is the type of 'effectful Text', the functions in Pipes.Text are 'pure' in the sense that they are uniformly monad-independent.

Pipes.Text.IO

Simple IO operations are defined in Pipes.Text.IO - as lazy IO Text operations are in Data.Text.Lazy.IO. It is generally

Pipes.Text.Encoding

In the text library, Data.Text.Lazy.Encoding handles inter-operation with Data.ByteString.Lazy. Here, Pipes.Text.Encoding provides for interoperation with the 'effectful ByteStrings' of Pipes.ByteString.

Implicit chunking

Remember that 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, but uses operations akin to those for strict text.

So also here: the operations in Pipes.Text are designed to operate on character streams that in a way that is independent of the boundaries of the underlying Text chunks. This means that they may freely split text into smaller texts and discard empty texts. The objective, though, is that they should not 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 (view, (%~)) -- or, Control.Lens

main = runEffect $ takeLines 3 Text.stdin >-> Text.stdout
  where 
    takeLines n = view Text.unlines . takes' n . view Text.lines
-- or equivalently: Text.unlines %~ takes' n

This program will not bring more into memory than what Text.stdin considers one chunk of text (~ 32 KB), even if individual lines are split across many chunks. The division into lines does not join Text fragments.

Lenses

As the use of view in 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; in particular it facilitates their use with Parsers of Text (in the general pipes-parse sense.) 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 called splitAt, 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 Pipes. Here too we recover the intuitively corresponding functions by prefixing them with (>->). Thus something like

 stripLines =  view 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 typically be the pipes equivalent of the function you think it is, given its name. So for example

view (Text.splitAt 300) :: Producer Text m r -> Producer Text (Producer Text m r)
Text.stdin ^. splitAt 300 :: Producer Text IO (Producer Text IO r) 

I.e., it produces the first 300 characters, and returns the rest of the producer. 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'

or equivalently:

upper n p = join (p ^. Text.splitAt n >-> Text.toUpper)

over / (%~)

If l is a Lens a b, over l is a function (b -> b) -> a -> a. Thus, given a function that modifies bs, the lens lets us modify an a by applying f :: b -> b to the b that we "see" in the a through the lens. So the type of over l f is a -> a for the concrete type 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 = over Text.lines (maps (>-> Text.stripStart))
stripLines = 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

The simple programs using the lines lens 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.