On 19/07/10 04:36, Michael Mossey wrote:
data Command = Play Int [Int] | Jump Int
-- I want to parse a string that will have any of the following forms -- and turn it into Command -- "p" - Produces "Play 0 []" -- "p v55" - Produces "Play 55 []" -- "p c123" - Produces "Play 0 [1,2,3]" -- "p v13 c12" - Produces "Play 13 [1,2]"
-- In other words the p command can have two kinds of arguments, "v" -- and "c", and there are defaults for the case that no argument is -- supplied. -- So it's going to look something like
play :: Parser Command play = do char 'p' vResult <- .. maybe a v, otherwise supply default value 0 cResult <- .. maybe a c .. (could the v and c be put in either order?) return $ Play vResult cResult
This isn't really an answer, but more of a suggestion on how to approach parsing problems. I tend to split things until I get down to easily handled stuff. In this case I'd probably write the following functions: 1. Parser for strings like "v55" and "v13": result type Parser Int 2. Parser for strings like "c123" and "c12": result type Parser [Int] 3. Parser combining 1 and 2: result type Parser (Int, [Int]) 4. Parser requiring a string starting with 'p ', combined with 3: result type Parser Command /M