prolog - 在 Prolog 中定义 "let expressions"

Question

在许多函数式编程语言中，可以使用 let 来“重新定义”局部变量。表达:

let example = 
    let a = 1 in
        let a = a+1 in
            a + 1

我找不到用于此目的的内置 Prolog 谓词，因此我尝试定义 let以这种方式表达:

:- initialization(main).
:- set_prolog_flag(double_quotes, chars).

replace(Subterm0, Subterm, Term0, Term) :-
        (   Term0 == Subterm0 -> Term = Subterm
        ;   var(Term0) -> Term = Term0
        ;   Term0 =.. [F|Args0],
            maplist(replace(Subterm0,Subterm), Args0, Args),
            Term =.. [F|Args]
        ).

let(A,B) :-
    ((D,D1) = (A1 is B1,C is B1);
    (D,D1) = (A1=B1,C=B1)),
    subsumes_term(D,A),
    D=A,
    replace(A1,C,B,B2),
    call((D1,B2)).

main :- let(A = 1,(
            writeln(A),
            let(A is A+1,(
                writeln(A),
                let(A is A * 2,(
                    writeln(A)
                ))
            ))
        )).

此实现似乎不正确，因为某些变量在被替换之前已绑定(bind)。我想定义一个允许同时“重新定义”多个变量的表达式:

main :- let((A = 1, B = 2), % this will not work with the let/2 predicate that I defined
            let((A=B,B=A),(
                writeln(A),
                writeln(B)
            ))  
        ).

是否可以实现 let以允许同时重新定义多个变量的方式表达？

Answer 1

定义 let 的问题作为一个正常的谓词，你不能重新定义出现在最外面的变量 let .这是我尝试使用目标扩展的更正确版本。 (对我来说这是有道理的，因为据我所知，在类似 lisp 的语言中，let 不能定义为函数，但可以定义为宏。)

%goal_expansion(let(Decl,OriginalGoal),Goal) :- %% SWI syntax
goal_expansion(let(Decl,OriginalGoal), _M, _, Goal, []) :- %%SICStus syntax 
        !,
        expand_let(Decl,OriginalGoal,Goal).
        
expand_let(X, OriginalGoal, Goal) :-
        var(X),
        !,
        replace(X,_Y,OriginalGoal,NewGoal),
        Goal=(true,NewGoal).        
expand_let(X is Decl, OriginalGoal, Goal) :-
        var(X),
        !,
        replace(X,Y,OriginalGoal,NewGoal),
        Goal=(Y is Decl,NewGoal).
expand_let(X = Decl, OriginalGoal, Goal) :-
        var(X),
        !,
        replace(X,Y,OriginalGoal,NewGoal),
        Goal=(Y = Decl,NewGoal).
expand_let([],OriginalGoal, Goal) :-
        !,
        Goal=OriginalGoal.
expand_let([L|Ls],OriginalGoal, Goal) :-
        !,
        expand_let_list([L|Ls],OriginalGoal,InitGoals,NewGoal),
        Goal=(InitGoals,NewGoal).
expand_let((L,Ls),OriginalGoal, Goal) :-
        !,
        expand_let(Ls,OriginalGoal, SecondGoal),
        expand_let(L,SecondGoal, Goal).

expand_let_list([],Goal,true,Goal).
expand_let_list([L|Ls],OriginalGoal,(Init,InitGoals),NewGoal):-
        (
          var(L)
        ->
          replace(L,_,OriginalGoal,SecondGoal),
          Init=true
        ;
          L=(X=Decl)
        ->
          replace(X,Y,OriginalGoal,SecondGoal),
          Init=(Y=Decl)
        ;
          L=(X is Decl)
        ->
          replace(X,Y,OriginalGoal,SecondGoal),
          Init=(Y is Decl)
        ),
        expand_let_list(Ls,SecondGoal,InitGoals,NewGoal).

这是重用 replace/4问题中定义的谓词。另请注意，钩子(Hook)谓词在 Prolog 版本之间有所不同。我正在使用 SICStus，它定义了 goal_expansion/5 .我快速浏览了文档，似乎 SWI-Prolog 有一个 goal_expansion/2 .
我在一个 let 中为多个声明引入了不同的语法。 : let((X1,X2),...)定义 X1 , 然后定义 X2 (所以相当于 let(X1,let(X2,...)) )，而 let([X1,X2],...)定义 X1和 X2同时(允许交换示例)。
以下是一些示例调用:

test1 :- let(A = 1,(
            print(A),nl,
            let(A is A+1,(
                print(A),nl,
                let(A is A + 1,(
                    print(A),nl
                ))
            ))
        )).

test2 :- A=2,let([A=B,B=A],(print(B),nl)).

test3 :- A=1, let((
                    A is A * 2,
                    A is A * 2,
                    A is A * 2
                  ),(
                      print(A),nl
                    )),print(A),nl.

test4 :- let([A=1,B=2],let([A=B,B=A],(print(A-B),nl))).

test5 :- let((
               [A=1,B=2],
               [A=B,B=A]
             ),(
                 print(A-B),nl
               )).