haskell - Parse a list whose separator may also occur at the end

Question

I am trying to parse some text, but I can't understand how to parse a list of symbols separated by some separator, which may or may not occur also at the end of the list.

Example (numbers separated by spaces):

set A = 1 2 3 4 5;
set B =6 7 8 9;
set C = 10 11 12 ;

If I use sepBy, after the last space I got an error because it expects another digit, even if I try to read also many whitespace after the list. If I use endBy, I got an error when the space is missing.

import Text.ParserCombinators.Parsec

main :: IO ()
main = do
  let input = "set A = 1 2 3 4 5;\n" ++
              "set B =6 7 8 9;\n" ++
              "set C = 10 11 12 ;\n"
  case parse parseInput "(unknown)" input of
    Left msg ->
      print msg
    Right rss ->
      mapM_ (\(n, vs) -> putStrLn (n ++ " = " ++ show vs)) rss

whitespace :: GenParser Char st Char
whitespace = oneOf " \t"

parseInput :: GenParser Char st [(String, [Int])]
parseInput = parseRow `endBy` newline

parseRow :: GenParser Char st (String, [Int])
parseRow = do
  string "set"
  many1 whitespace
  name <- many1 alphaNum
  many whitespace
  string "="
  many whitespace
  values <- many1 digit `sepBy` many1 whitespace
  many whitespace
  string ";"
  return (name, map read values)

Answer 1

我认为你想要的组合器是sepEndBy. 使用它给你

-- I'm using the type synonym
-- type Parser = GenParser Char ()
-- from Text.ParseCombinator.Parsec.Prim
parseRow :: Parser (String, [Int])
parseRow = do
  string "set" >> many1 whitespace
  name <- many1 alphaNum
  spaces >> char '=' >> spaces
  values <- many1 digit `sepEndBy` many1 whitespace
  char ';'
  return (name, map read values)
  where spaces = many whitespace