序言，写没有原子的表达 [英] Prolog, writing expression with no atoms

问题描述

Prolog的新手，我希望能够在很好地了解发生了什么情况的同时回答这个问题

New to Prolog and I want to be able to answer this question while getting a good understanding of whats going on

写一个谓词no_atoms/1，如果它的参数是一个没有任何原子.例如，no_atoms(5-(3 + x))应该失败，no_atoms((12 + 8)-7)应该会成功.

Write a predicate no_atoms/1 which succeeds iff its argument is an expression without any atoms. For example, no_atoms(5-(3+x)) should fail, no_atoms((12+8)-7) should succeed.

任何帮助将不胜感激！

推荐答案

您需要测试表达式不是原子.然后，使用compound/1进行检查，是否需要使用arg/3枚举参数，或者是否需要成功.

You need to test that the expression is not an atom. Then, you check with compound/1 if you need to enumerate the arguments with arg/3 or succeed.

no_atoms(Expr) :-
    \+ atom(Expr),
    (   compound(Expr)
    ->  \+ ( arg(_, Expr, A), \+ no_atoms(A) )
    ;   true
    ).

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