Parse an integer in the octal representation. The format of the octal
number is expected to be according to the Haskell report except for
the fact that this parser doesn't parse “0o” or “0O” prefix. It is a
responsibility of the programmer to parse correct prefix before
parsing the number itself.
For example you can make it conform to the Haskell report like this:
octal = char '0' >> char' 'o' >> L.octal
Note: before version
6.0.0 the function returned
Integer, i.e. it wasn't polymorphic in its return type.
Warning: this function does not perform range checks.