parsec: string choice parser with useful error messages

Question

Let's have following parser:

parser :: GenParser Char st String
parser = choice (fmap (try . string) ["head", "tail", "tales"]
                    <?> "expected one of ['head', 'tail', 'tales']")

When we parse the malformed input "ta" it will return the defined error but because of backtracking it will also talk about unexpected "t" at first position instead of unexpected " " at position 3.

Is there an easy (or built-in) way of matching one of multiple expected strings that produces good error messages? I am talking about showing the correct position and in this case something like expected "tail" or "tales" instead of our hard-coded error message.

Answer 1

It's not hard to cook up a function which does this correctly. We'll just rip one character off at a time, using Data.Map to find the shared suffixes:

{-# LANGUAGE FlexibleContexts #-}
import Control.Applicative
import Data.Map hiding (empty)
import Text.Parsec hiding ((<|>))
import Text.Parsec.Char

-- accept the empty string if that's a choice
possiblyEmpty :: Stream s m Char => [String] -> ParsecT s u m String
possiblyEmpty ss | "" `elem` ss = pure ""
                 | otherwise    = empty

chooseFrom :: Stream s m Char => [String] -> ParsecT s u m String
chooseFrom ss
     =  foldWithKey (\h ts parser -> liftA2 (:) (char h) (chooseFrom ts) <|> parser)
                    empty
                    (fromListWith (++) [(h, [t]) | h:t <- ss])
    <|> possiblyEmpty ss

We can verify in ghci that it succesfully matches "tail" and "tales", and that it asks for i or l after a failed parse starting with ta:

*Main> parse (chooseFrom ["head", "tail", "tales"]) "" "tail"
Right "tail"
*Main> parse (chooseFrom ["head", "tail", "tales"]) "" "tales"
Right "tales"
*Main> parse (chooseFrom ["head", "tail", "tales"]) "" "tafoo"
Left (line 1, column 3):
unexpected "f"
expecting "i" or "l"