--- /dev/null
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
+
+module Pipes.Text.Tutorial (
+ -- * Effectful Text
+ -- $intro
+ -- ** @Pipes.Text@
+ -- $pipestext
+ -- ** @Pipes.Text.IO@
+ -- $pipestextio
+ -- ** @Pipes.Text.Encoding@
+ -- $pipestextencoding
+ -- * 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 /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.
+
+ 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>
+ (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.
+ Simple /IO/ operations are defined in @Pipes.Text.IO@ - as lazy IO @Text@
+ operations are in @Data.Text.Lazy.IO@. Similarly, as @Data.Text.Lazy.Encoding@
+ handles inter-operation with @Data.ByteString.Lazy@, @Pipes.Text.Encoding@ provides for
+ interoperation with the \'effectful ByteStrings\' of @Pipes.ByteString@.
+
+ 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 functions in this module 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)
+>
+> main = runEffect $ takeLines 3 Text.stdin >-> Text.stdout
+> where
+> takeLines n = view Text.unlines . takes' n . view Text.lines
+
+ This program will never bring more into memory than what @Text.stdin@ considers
+ one chunk of text (~ 32 KB), even if individual lines are split across many chunks.
+
+-}
+{- $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 '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 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 = 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.drop)
+ > 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'
+-}
+{- $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.
+
+-}
projects / github / fretlink / text-pipes.git / blobdiff