Codes is the codes comprising the printed representation of Number.
Initially, either Number must be instantiated to a number, or Codes must be instantiated to a proper codes.
If Codes is instantiated to a codes that corresponds to the correct syntax of a number, Number will be unified with that number.
Else, Number should be instantiated to a number, and Codes will be unified with the codes that make up its printed representation.
The check of Codes when Number is instantiated was added in release 4.3 for alignment with the ISO Prolog standard. Previous releases simply failed in this case instead of reporting an error for malformed Codes.
| ?- number_codes(foo, L). ! Type error in argument 1 of number_codes/2 ! expected a number, but found foo ! goal: number_codes(foo,_104)
| ?- number_codes(431, L). L = [52,51,49]
| ?- number_codes(X, [102,111,111]). ! Syntax error in number_codes/2 ! number syntax ! in line 0
| ?- number_codes(X, [52,51,49]). X = 431
| ?- number_codes(X, "15.0e+12"). X = 1.5E+13