• If CharList is a proper list, i.e., not unbound or a partial list, CharList is parsed according to the Prolog syntax for numbers and the resulting number is unified with Number. A syntax_error exception is raised if CharList is instantiated to a ground, proper list but does not represent a valid Prolog number.
• Otherwise, if Number is indeed a number, Number is serialized and the result is unified with CharList.

Following the ISO standard, the Prolog syntax for number allows for leading white space (including newlines) and does not allow for trailing white space.^{120ISO also allows for Prolog comments in leading white space. We--and most other implementations--believe this is incorrect. We also believe it would have been better not to allow for white space, or to allow for both leading and trailing white space.}

Beyond ASCII, decimal digits in the input may come from any Unicode decimal digit block (general category Nd); see section 2.15.1.9. All digits in a single number must come from the same block --- for an integer, for the numerator and denominator of a rational, and for the mantissa and exponent of a floating-point number. The optional sign (+, -), the rational separator (r or /), the floating-point decimal point (.) and the floating-point exponent letter (e, E) are always ASCII; their non-ASCII look-alikes raise syntax_error.

Prolog syntax-based conversion can also be achieved using format/3 and read_from_chars/2.