pipes-text 0.0.0.14 → 0.0.0.15

raw patch · 2 files changed

+312/−2 lines, 2 filesPVP ok

version bump matches the API change (PVP)

API changes (from Hackage documentation)

Files

• Pipes/Text/Tutorial.hs +310/−0
• pipes-text.cabal +2/−2

@@ -0,0 +1,310 @@+{-# 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.++-}

@@ -1,5 +1,5 @@ name:                pipes-text-version:             0.0.0.14+version:             0.0.0.15 synopsis:            Text pipes. description:         * This package will be in a draft, or testing, phase until version 0.0.1. Please report any installation difficulties, or any wisdom about the api, on the github page or the <https://groups.google.com/forum/#!forum/haskell-pipes pipes list>                      .@@ -50,5 +50,5 @@   ghc-options: -O2    if !flag(noio)-    exposed-modules:   Pipes.Text.IO+    exposed-modules:   Pipes.Text.IO, Pipes.Text.Tutorial     build-depends:     text >=0.11.3              && < 1.3