How to parse an Integer with parsec

问题

I was expecting to find a function

integer :: Stream s m Char => ParsecT s u m Integer

or maybe even

natural :: Stream s m Char => ParsecT s u m Integer

in the standard libraries, but I did not find one.

What is the standard way of parsing plain natural numbers directly to an Integer?

回答1:

Here is what I often do is to use the expression

read <$> many1 digit

which can have type Stream s m Char => ParsecT s u m Integer (or simply Parser Integer).

I don’t like the use of the the partial function read, but when the parser succeeds I know that the read will succeed, and it is somewhat readable.

回答2:

Looking at the source of Text.Parsec.Token, it seems Parsec doesn't have a dedicated function for it. They do give a default definition for the decimal field of GenLanguageDef. decimal is defined similar to:

decimal = do
    digits <- many1 baseDigit
    let n = foldl (\x d -> base*x + toInteger (digitToInt d)) 0 digits
    seq n (return n)
  where
    base = 10
    baseDigit = digit

Here, digit is taken from Text.Parsec.Char and digitToInt from Data.Char.

There's also a default definition for natural, which, by default, also parses octal and hexadecimal numbers, and skips trailing whitespace.

来源：https://stackoverflow.com/questions/24171005/how-to-parse-an-integer-with-parsec