Parser Combinator Test

ParserCombinator.purs

module Main where

import Prelude
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (log, CONSOLE)
import Control.Plus ((<|>))
import Data.Maybe (Maybe(..))
import Text.Parsing.StringParser (Parser, runParser)
import Text.Parsing.StringParser.Combinators (optionMaybe)
import Text.Parsing.StringParser.String (string)

main :: ∀ e. Eff (console :: CONSOLE | e) Unit
main = do
  -- This one finds the `noColon` first
  -- Output: (Right (Just "test"))
  logParse ((Just <$> noColon) <|> colon) "test"

  -- This one fails on the `colon` and never tries the `noColon` branch
  -- Output: (Left Expected ':'.)
  -- Expected: (Right Nothing)
  logParse (colon <|> (Just <$> noColon)) "test"

  where
    noColon :: Parser String
    noColon = string "test"

    -- I would expect this function to return `Parser Nothing`
    -- in the event that we couldn't find the trailing ':', but
    -- it seems to `fail` with an error inside the Parser instead.
    colon :: Parser (Maybe String)
    colon = optionMaybe $ (string "test" <* string ":")


    logParse p s = log $ show $ runParser p s