Checking if the remaining input is whitespace with parsec

Question

I'm trying out the parsec library and I'm not sure how to handle this basic task.

Suppose I have the following:

data Foo = A | AB

and I want the string "a" to be parsed as A and "a b" AB. If I just do this:

parseA :: parser Foo
parseA = do
           reserved "a"
           return A

parseAB :: parser Foo
parseAB = do
            reserved "a"
            reserved "b"
            return AB

parseFoo :: parser Foo
parseFoo =  parseA
        <|> parseAB

then parseFoo will parse "a b" as A since parseA doesn't care that there is non-whitespace still left after consuming the 'a'. How can this be fixed?

Answer 1

You need to change grammar to AB | A and use try from parsec, which gives lookahead capability to your parser.

This should work

parseFoo = try Parse AB <|> parse A