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parsinghaskellparsec

Parsing issue with parens. Parsec - Haskell

This is my code:

expr :: Parser Integer
expr = buildExpressionParser table factor <?> "expression"

table :: [[ Operator Char st Integer ]]
table = [
    [ op "*" (*) AssocLeft],
    [ op "+" (+) AssocLeft]
    ]
  where
    op s f assoc = Infix (do { string s ; return f }) assoc

factor = do { char '(' ; x <- expr ; char ')' ; return x }
    <|> number
    <?> "simple expression"


number :: Parser Integer
number = do { ds <- many1 digit; return read(ds))) } <?> "number"

This works perfectly with expressions like this: (10+10) * 10. But I have a problem with the following: 10 +10) This has to return a parsing error (just 1 parenthesis at the end), but instead it returns 20.

How to fix this?

Thanks!

Solution

  • You'll need to use the eof parser to make sure you've read the whole input:

    myParser = do
    parsed <- expr
    eof
    return parsed

    If you're using Control.Applicative this can be simplified to:

    myParser = expr <* eof