{-# OPTIONS_GHC -fno-warn-unused-imports #-} module Pipes.Text.Tutorial ( -- * Effectful Text -- $intro -- * Lenses -- $lenses -- ** @view@ \/ @(^.)@ -- $view -- ** @over@ \/ @(%~)@ -- $over -- ** @zoom@ -- $zoom -- * Special types: @Producer Text m (Producer Text m r)@ and @FreeT (Producer Text m) m r@ -- $special ) where import Pipes import Pipes.Text import Pipes.Text.IO import Pipes.Text.Encoding {- $intro 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. The main @Pipes.Text@ module provides many functions equivalent in one way or another to the pure functions in <https://hackage.haskell.org/package/text-1.1.0.0/docs/Data-Text-Lazy.html 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 <http://hackage.haskell.org/package/pipes-parse-3.0.1/docs/Pipes-Parse-Tutorial.html 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 <http://hackage.haskell.org/package/lens lens> and <http://hackage.haskell.org/package/lens-family lens-family> The use of 'zoom' is explained in <http://hackage.haskell.org/package/pipes-parse-3.0.1/docs/Pipes-Parse-Tutorial.html Pipes.Parse.Tutorial> and to some extent in the @Pipes.Text.Encoding@ module here. -} {- $special 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. -}