parsing - Prolog 中的 DCG——字符串

Question

我正在使用 Prolog 的内置 DCG 功能编写一个 Lisp-to-C 转换器。这就是我处理算术的方式:

expr(Z) --> "(", "+", spaces, expr(M), spaces, expr(N), ")", {swritef(Z, "%d + %d", [M, N])}.
expr(Z) --> "(", "-", spaces, expr(M), spaces, expr(N), ")", {swritef(Z, "%d - %d", [M, N])}.
expr(Z) --> "(", "*", spaces, expr(M), spaces, expr(N), ")", {swritef(Z, "%d * %d", [M, N])}.
expr(Z) --> "(", "/", spaces, expr(M), spaces, expr(N), ")", {swritef(Z, "%d / %d", [M, N])}.
expr(E) --> number(E).

number(C) --> "-", digits(X), {C is -X}.
number(C) --> digits(C).
digits(D) --> digit(D);digit(A),digits(B), {number_codes(B,Cs),length(Cs,L), D is A*(10^L)+B}.
digit(D) --> [C], {"0"=<C, C=<"9", D is C - "0"}.

就像现在一样，它不处理嵌套表达式。这是我认为可行的方法:

expr(Z) --> "(", "+", spaces, expr(M), spaces, expr(N), ")", {swritef(Z, "%s + %s", [M, N])}.
expr(E) --> number(N), {swritef(E, "%d", [N])}.

但是我得到了这个:

?- expr(E, "42", []).
E = "42" %all OK

?- expr(E, "(+ 3 (* 2 2))", []).
E = "%s + %s" %not OK

如何让它发挥作用？

Answer 1

问题是 %s 格式说明符需要参数是字符列表。所以你可以这样做:

:-set_prolog_flag(double_quotes, codes).  % This is for SWI 7+ to revert to the prior interpretation of quoted strings.

expr(Z) --> "(", "+", spaces, lexpr(M), spaces, lexpr(N), ")", {swritef(Z, "%s + %s", [M, N])}.
expr(Z) --> "(", "-", spaces, lexpr(M), spaces, lexpr(N), ")", {swritef(Z, "%s - %s", [M, N])}.
expr(Z) --> "(", "*", spaces, lexpr(M), spaces, lexpr(N), ")", {swritef(Z, "%s * %s", [M, N])}.
expr(Z) --> "(", "/", spaces, lexpr(M), spaces, lexpr(N), ")", {swritef(Z, "%s / %s", [M, N])}.
expr(N) --> number(N).

lexpr(Z) --> expr(M), {atom_chars(M, Z)}.

number(C) --> "-", digits(X), {C is -X}.
number(C) --> digits(C).

digits(D) --> digit(D);digit(A),digits(B), {number_codes(B,Cs),length(Cs,L), D is A*(10^L)+B}.
digit(D) --> [C], {"0"=<C, C=<"9", D is C - "0"}.

spaces --> " ", spaces.
spaces --> [].

谓词 lexpr 只是将解析后的表达式转换为字符列表。

编辑:03/07/2016:从 SWI 7.0 版开始，双引号中的文本不再被解释为字符代码列表。您可以使用反引号 (`) 更改双引号或添加指令；

:-set_prolog_flag(double_quotes, codes).

在代码的开头。