Haskell Text.Parsec.Combinator choice doesn't backtrack

Question

I'm trying to parse some Text with parsec:

data Cmd = LoginCmd String
         | JoinCmd String
         | LeaveCmd String
    deriving (Show)

singleparam :: Parser Cmd
singleparam = do
    cmd <- choice [string "leave", string "login", string "join"]
    spaces
    nick <- many1 anyChar
    eof
    return $ LoginCmd nick

I'm expecting choice to try to match "leave", and if it fails, then try "login" etc. But it only tries to match "leave", and if it fails, then gives an error.

ghci> parseTest singleparam (pack "login asdf")
parse error at (line 1, column 1):
unexpected "o"
expecting "leave"
ghci> parseTest singleparam (pack "leave asdf")
LoginCmd "asdf"

What am I doing wrong?

score 17 · Accepted Answer · answered Apr 02 '12 at 12:34

Parsec does not automatically backtrack like this (for efficiency). The rule is that once a branch accepts a token then alternative branches are pruned. The solution is to add an explicit backtracking using try (string "leave") and try (string "login") etc.

In your example the 'l' character is the token that commits Parsec to the first "leave" branch and abandons the next branches of "login" and "join".

More detail in Real World Haskell (book, online) on parsec.

Haskell Text.Parsec.Combinator choice doesn't backtrack

1 Answers1