Prolog的DCG问题

Question

该项目是关于将半自然语言翻译为SQL表格的。代码：

label(S) --> label_h(C), {atom_codes(A, C), string_to_atom(S, A)}, !. 

label_h([C|D]) --> letter(C), letters_or_digits(D), !. 

letters_or_digits([C|D]) --> letter_or_digit(C), letters_or_digits(D), !. 
letters_or_digits([C]) --> letter_or_digit(C), !. 
letters_or_digits([]) --> "", !. 

letter(C) --> [C], {"a"=<C, C=<"z"}, !. 
letter(C) --> [C], {"A"=<C, C=<"Z"}, !. 
letter_or_digit(C) --> [C], {"a"=<C, C=<"z"}, !. 
letter_or_digit(C) --> [C], {"A"=<C, C=<"Z"}, !. 
letter_or_digit(C) --> [C], {"0"=<C, C=<"9"}, !. 

table("student"). 

sbvr2sql --> label(Name), " is an integer.", {assert(fields(Name, "INT"))}. 
sbvr2sql --> label(Name), " is a string.", {assert(fields(Name, "VARCHAR(64)"))}. 

sbvr2sql(Table, Property) --> label(Table), " has ", label(Property), ".". 

这里是如何工作的罚款：

?- sbvr2sql("age is an integer.", []). 
true 

?- sbvr2sql("firstName is a string.", []). 
true. 

?- sbvr2sql(T, P, "student has firstName.", []). 
T = "student", 
P = "firstName". 

?- fields(F, T). 
F = "age", 
T = [73, 78, 84] n 
F = "firstName", 
T = [86, 65, 82, 67, 72, 65, 82, 40, 54|...]. 

?- sbvr2sql(T, P, "student has firstName.", []), fields(P, _). 
T = "student", 
P = "firstName". 

但它不会在这里工作：

?- table(T). 
T = [115, 116, 117, 100, 101, 110, 116]. % "student" 

?- sbvr2sql(T, P, "student has firstName.", []), table(T). 
false. 

显然，它不承认table("student")为真。它将“学生”视为上述标签。是什么赋予了？

Answer 1

我无法重现该错误，但我怀疑它可能在您的label/3规则中。当我用这个规则的定义如下：

label([C|S]) --> 
    [C], {[Sp|_] = " ", C \= Sp, [Dot|_] = ".", C \= Dot}, !, 
    label(S). 
label([],X,X). 

我得到正确的结果：

?- sbvr2sql(TS, PS, "student has firstName.", []), table(TS), 
    atom_codes(P,PS), atom_codes(T,TS). 
TS = [115, 116, 117, 100, 101, 110, 116], 
PS = [102, 105, 114, 115, 116, 78, 97, 109, 101], 
P = firstName, 
T = student. 

在一般情况下，我建议你做DCG操作前标化字符串成原子的名单。这样，由于Prolog的尴尬字符串输出，调试更容易。