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+{-# 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.
+
+-}