求助：实现输入±计算3±2的Prolog程序编译报错，如何解决？

Let's break down why your current code is failing and how to fix it properly.

Why Your Original Code Errors #

Your code:

try(B):- read(A), B is A(3,2).

Runs into two main issues:

1. Input Parsing Problem: When you type + or - directly, Prolog's read/1 can't interpret it as a valid standalone term—these are infix operators, not atomic values. You'd have to input them as quoted atoms (like '+') just to get read/1 to accept them, which is not user-friendly.
2. Arithmetic Expression Issue: Even if A is correctly read as '+', B is A(3,2) doesn't work as expected in most Prolog implementations. The is/2 predicate expects an arithmetic expression (like 3+2), not a functor call syntax like +(3,2) (though they're logically equivalent, is/2 doesn't always handle the latter smoothly).

Working Solutions #

Solution 1: Explicit Conditional Checks (Simple & User-Friendly) #

This approach directly handles each operator case and guides the user if they enter something invalid:

calculate_result(Result) :-
    write('Enter operator (+ or -): '),
    read_term(Op, [variable_names([])]),  % Reads operator correctly even without quotes
    (   Op = (+)
    ->  Result is 3 + 2
    ;   Op = (-)
    ->  Result is 3 - 2
    ;   write('Oops! Please enter either + or -.'), nl,
        calculate_result(Result)  % Retry on invalid input
    ).

• Using read_term/2 instead of read/1 lets users input + or - directly without quotes, which is much more intuitive.
• The conditional branches (->) explicitly map each operator to the correct arithmetic calculation.

Solution 2: Dynamic Expression Construction (More Flexible) #

If you want to handle operators dynamically (scaling to more operators later), use the =../2 (univ) predicate to build the arithmetic expression:

calculate_result(Result) :-
    write('Enter operator (+ or -): '),
    read_term(Op, [variable_names([])]),
    (   member(Op, [+, -])
    ->  Expr =.. [Op, 3, 2],  % Builds e.g., +(3,2) when Op is +
        Result is Expr
    ;   write('Invalid operator! Try + or -.'), nl,
        calculate_result(Result)
    ).

• =../2 takes a functor and arguments and turns them into a term (or vice versa). Here, it constructs the arithmetic expression from the input operator, then is/2 evaluates it.

How to Test #

Call the predicate in your Prolog interpreter:

?- calculate_result(R).
Enter operator (+ or -): +
R = 5.

?- calculate_result(R).
Enter operator (+ or -): -
R = 1.

内容的提问来源于stack exchange，提问作者Qwerto