问题
I'm trying to specify a DCG for a valid number that would be used like so:
value(Number) --> valid_number(Number).
Basically checking if a specified value is numeric, (it could also be a variable, so it's necessary to check).
I don't know how to build this valid_number DCG/predicate though.
Right now I just have:
valid_number('1') --> ['1'].
valid_number('2') --> ['2'].
...
Which works but it obviously terrible. Trying something like:
valid_number(Number) --> { integer(Number), Number = Number }.
Which both doesn't work and admittedly looks pretty gross as well (I'm sorry, very new to Prolog and trying to learn best practices).
How would I go about building this DCG/predicate that validates whether or not it's a number?
回答1:
I give you a code sample that describes natural numbers:
:- set_prolog_flag(double_quotes, chars).
natural_number(N) --> number_(Cs), { number_codes(N, Cs) }.
number_([D|Ds]) --> digit(D), number_(Ds).
number_([D]) --> digit(D).
digit(D) --> [D], { char_type(D, digit) }.
Example usage:
?- phrase(natural_number(N), "123").
N = 123 ;
false.
I leave generalizing this to other numbers as an exercise.
回答2:
Here's an ABNF grammar for a numeric literal such as you might find in a computer language:
NUMERIC-CONSTANT = INTEGER-PART 0*1(FRACTIONAL-PART) 0*1(EXPONENT)
INTEGER-PART = 0*1(SIGN) 1*(DIGIT)
FRACTIONAL-PART = 0*1( DECIMAL-POINT 1*(DIGIT) )
EXPONENT = EXP 0*1(SIGN) 1*(DIGIT)
EXP = "E" / "e"
DECIMAL-POINT = "."
SIGN = "+" / "-"
DIGIT = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"
Since there's no left recursion in this grammar, this lends itself directly to a recursive descent parser, such as produced by a Prolog DCG:
numeric_constant --> integer , optional_fraction , optional_exponent .
integer --> optional_sign , digits .
optional_sign --> sign .
optional_sign --> [].
digits --> digit , digits .
optional_fraction --> fraction .
optional_fraction --> [].
fraction --> decimal_point , digits .
optional_exponent --> exponent .
optional_exponent --> [].
exponent --> exp , optional_sign , digits .
exp --> ['E'] .
exp --> ['e'] .
decimal_point --> ['.'] .
sign --> ['+'] .
sign --> ['-'] .
digit --> ['0'] .
digit --> ['1'] .
digit --> ['2'] .
digit --> ['3'] .
digit --> ['4'] .
digit --> ['5'] .
digit --> ['6'] .
digit --> ['7'] .
digit --> ['8'] .
digit --> ['9'] .
来源：https://stackoverflow.com/questions/32870994/how-do-i-specify-a-dcg-for-a-valid-number