The solution I came up with was to push as much logic into the parser as
possible:
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Applicative
import qualified Data.ByteString as B
import Pipes
import qualified Pipes.Prelude as P
import qualified Pipes.Attoparsec as P
import Data.Attoparsec.ByteString.Char8 as A
data Foo = Foo deriving Show
data Bar = Bar deriving Show
data Baz = Baz deriving Show
data Command = Command Foo Bar Baz deriving Show
data Escape = Escape deriving Show
wrap :: Parser a -> Parser (Maybe a)
wrap p
= fmap Just p
<|> ("escape" *> pure Nothing)
<|> anyChar *> wrap p
fooParser :: Parser Foo
fooParser = string "foo" >> return Foo
barParser :: Parser Bar
barParser = string "bar" >> return Bar
bazParser :: Parser Baz
bazParser = string "baz" >> return Baz
exitParser :: Parser Escape
exitParser = string "escape" >> return Escape
commandParser :: Parser (Maybe Command)
commandParser =
liftA3 (liftA3 Command)
(wrap fooParser)
(wrap barParser)
(wrap bazParser)
example :: Producer B.ByteString IO ()
example = do
yield "foobarbaz"
yield "foo"
yield "bar"
yield "invalid" -- skip over invalid input
yield "baz"
yield "foo"
yield "escape" -- exit early from parse based on specific value
yield "foobarbaz"
main :: IO ()
main = do
runEffect $ P.parsed commandParser example >-> P.print
return ()
This gives the following output:
Just (Command Foo Bar Baz)
Just (Command Foo Bar Baz)
Nothing
Then you can tell the downstream stages to stop looping when you get a
`Nothing` since that indicates that it parsed the `escape` token.
On 10/23/2015 10:30 PM, Michael Whitehead wrote:
|
{-# LANGUAGE OverloadedStrings #-}
module Main where
import qualified Data.ByteString as B
import Pipes
import qualified Pipes.Prelude as P
import qualified Pipes.Attoparsec as P
import Data.Attoparsec.ByteString.Char8 as A
data Foo = Foo deriving Show
data Bar = Bar deriving Show
data Baz = Baz deriving Show
data Command = Command Foo Bar Baz deriving Show
data Escape = Escape deriving Show
fooParser :: Parser Foo
fooParser = string "foo" >> return Foo
barParser :: Parser Bar
barParser = string "bar" >> return Bar
bazParser :: Parser Baz
bazParser = string "baz" >> return Baz
exitParser :: Parser Escape
exitParser = string "escape" >> return Escape
commandParser :: Parser Command
commandParser =
Command <$> fooParser
<*> barParser
<*> bazParser
example :: Producer B.ByteString IO ()
example = do
yield "foobarbaz"
yield "foo"
yield "bar"
yield "invalid" -- skip over invalid input
yield "baz"
yield "foo"
yield "escape" -- exit early from parse based on specific value
yield "foobarbaz"
main :: IO ()
main = do
runEffect $ P.parsed commandParser example >-> P.print
return ()
|
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