The Data.Char module contains helper functions that determine whether a Char is a decimal, octal or hexadecimal digit (isDigit, isOctDigit, isHexDigit). A similar helper function for binary digits is not included for no apparent reason (maybe because the number parsers in the Numeric module also don't support binary digits?).

The implementation for such a isBinDigit should be trivial, aside from performance considerations: isDigit and friends perform the test using only subtraction and an unsigned comparison, I'm not sure whether isBinDigit offers any different performant implementations.