From 80a490ef5673cd22586215732bf8f596437e8f59 Mon Sep 17 00:00:00 2001 From: michaelt Date: Tue, 25 Feb 2014 01:12:15 -0500 Subject: Pipes.Text documentation approaching tutorial length --- Pipes/Text.hs | 152 +++++++++++++++++++++++++++++++++++++++++++++---------- Pipes/Text/IO.hs | 4 +- 2 files changed, 127 insertions(+), 29 deletions(-) (limited to 'Pipes') diff --git a/Pipes/Text.hs b/Pipes/Text.hs index 95fc0e6..9f84429 100644 --- a/Pipes/Text.hs +++ b/Pipes/Text.hs @@ -135,7 +135,7 @@ import Prelude hiding ( {- $intro - * /Effectful Text/ + * /I. Effectful Text/ This package provides @pipes@ utilities for /text streams/, understood as streams of 'Text' chunks. The individual chunks are uniformly /strict/, and thus you @@ -178,36 +178,103 @@ import Prelude hiding ( The above program will never bring more than one chunk of text (~ 32 KB) into memory, no matter how long the lines are. - * /Lenses/ + * /II. 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 - - sense.) - Each such lens, e.g. 'lines', 'chunksOf' or 'splitAt', reduces to the - intuitively corresponding function when used with @view@ or @(^.)@. + 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 + 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 + + > producer ^. splitAt 17 - Note similarly that many equivalents of 'Text -> Text' functions are exported here as 'Pipe's. - They reduce to the intuitively corresponding functions when used with '(>->)'. Thus something like + 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 lens combinators - you will find indispensible are @view@ / @(^.)@), @zoom@ and probably @over@. These - are supplied by both and + 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 "focussing". + 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 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 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 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 8) drawAll) p +> let seven = T.concat ts +> case T.toUpper seven of +> "TOUPPER" -> p' >-> Text.toUpper +> "TOLOWER" -> p' >-> Text.toLower +> _ -> do yield seven +> p' + + 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 and The use of 'zoom' is explained in - and to some extent in the @Pipes.Text.Encoding@ module here. The use of - @over@ is simple, illustrated by the fact that we can rewrite @stripLines@ above as + and to some extent in the @Pipes.Text.Encoding@ module here. -> stripLines = over Text.lines $ maps (>-> stripStart) - - * Special types: @Producer Text m (Producer Text m r)@ and @FreeT (Producer Text m) m r@ + * /III. Special types:/ @Producer Text m (Producer Text m r)@ /and/ @FreeT (Producer Text m) m r@ 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, @@ -259,16 +326,23 @@ import Prelude hiding ( > (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, @FreeT (Producer Text m) m r@ - encompasses all the members of the sequence: + (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 -> Producer Text m r -> Producer Text m (Producer Text m r) -> Producer Text m (Producer Text m (Producer Text 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) @@ -295,9 +369,33 @@ import Prelude hiding ( 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. + +-} -- | Convert a lazy 'TL.Text' into a 'Producer' of strict 'Text's fromLazy :: (Monad m) => TL.Text -> Producer' Text m () diff --git a/Pipes/Text/IO.hs b/Pipes/Text/IO.hs index 101052b..627582e 100644 --- a/Pipes/Text/IO.hs +++ b/Pipes/Text/IO.hs @@ -33,9 +33,9 @@ import Pipes.Safe (MonadSafe(..), Base(..)) import Prelude hiding (readFile, writeFile) {- $textio - Where pipes IO replaces lazy IO, @Producer Text m r@ replaces lazy 'Text'. + Where pipes @IO@ replaces lazy @IO@, @Producer Text IO r@ replaces lazy 'Text'. This module exports some convenient functions for producing and consuming - pipes 'Text' in IO, namely, 'readFile', 'writeFile', 'fromHandle', 'toHandle', + pipes 'Text' in @IO@, namely, 'readFile', 'writeFile', 'fromHandle', 'toHandle', 'stdin' and 'stdout'. Some caveats described below. The main points are as in -- cgit v1.2.3