Nevermind. If I change the wrap function then I get what I want.
wrap :: Parser a -> Parser a
wrap p = p
<|> ("escape" *> p)
<|> anyChar *> wrap p
Thanks for the help!
On Monday, October 26, 2015 at 8:54:06 AM UTC-6, Michael Whitehead wrote:
>
> > The solution I came up with was to push as much logic into the parser as
> possible:
>
> Thanks for your solution. That is pretty much what I was looking for
>
> Just one last thing that I am trying to figure out is that the command
> that immediately follows an "escape" gets swallowed somehow.
>
> So in this example
>
> example :: Producer B.ByteString IO ()
> example = do
> yield "foobarbaz"
> yield "foo"
> yield "escape" -- exit early from parse based on specific value
> yield "foobarbaz"
>
> It should give
>
> Just (Command Foo Bar Baz)
> Nothing
> Just (Command Foo Bar Baz)
>
> But it only gives this
>
> Just (Command Foo Bar Baz)
> Nothing
>
> I know that there is probably something really simple that I am missing.
>
>
> On Sunday, October 25, 2015 at 3:45:47 PM UTC-6, Gabriel Gonzalez wrote:
>>
>> 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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