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  1. PO
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    <p>No, because <code>=/2</code> does not mean <em>assign</em> in Prolog, but rather <em>unify</em>. And the unification algorithm does not know anything at all about arithmetic, just structure. So you can do some interesting things with arithmetic expressions in Prolog that are quite difficult to pull off in other languages:</p> <pre><code>?- X = 5 + 2. X = 5+2. </code></pre> <p>It looks like nothing happened there, but what actually happened is X was given the value "5 + 2" <em>as a structure</em>. Put another way:</p> <pre><code>?- A + B = 5 + 2. A = 5, B = 2. </code></pre> <p>Or even: </p> <pre><code>?- X = 5 + 2, X =.. [Op|_]. X = 5+2, Op = (+). </code></pre> <p>That last one might make more sense with the whole list though:</p> <pre><code>?- X = 5 + 2, X =.. Y. X = 5+2, Y = [+, 5, 2]. </code></pre> <p>This is an effect of the remarkable "univ" operator, <code>=../2</code>, which is able to convert between Lisp-like lists and Prolog syntax, enabling you to do interesting construction and decomposition of structures in a generic fashion.</p> <p>Now, <code>is/2</code>, on the other hand, <em>does</em> know about arithmetic. It unifies its left argument with the result of arithmetic simplification of its right. <strong>Do note</strong> that it only works in one direction:</p> <pre><code>?- 7 is 5 + 2. true. ?- 5 + 2 is 7. false. </code></pre> <p>You could say that <code>=/2</code> is interested in structure and <code>is/2</code> is interested in numeric equality. But it does mean that it's unusually easy to teach Prolog algebra:</p> <pre><code>simplify(X * Y + X * Z, X * (Y + Z)). % distributive property simplify(X, X). ?- simplify(A * 3 + A * 4, Q). Q = A* (3+4) </code></pre> <p>Now, this isn't perfect (note that we got 3+4 and not 7) and it's still a lot of work to make it really smart:</p> <pre><code>?- simplify(3 * A + 4 * A, Q). Q = 3*A+4*A. </code></pre> <p>But that's a problem for another day.</p> <p>Nutshell:</p> <ul> <li><code>=/2</code> triggers unification</li> <li><code>is/2</code> triggers arithmetic evaluation</li> </ul>
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    1. POWhat is the difference between is and =?
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    1. USDaniel Lyons
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    1. POWhat is the difference between is and =?
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    1. COThank you, especially for the nutshell! I'm curious why you put /2 after = and is in your answer?
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      1. USDanny Rancher
    2. COThe slash separates the name of the predicate from its "arity," the number of arguments it accepts. This will only ever be 2 for infix operators like 'is' and '=', but for other predicates you can use the same name with versions that take different numbers of arguments, so you need this to keep your `foo/2` straight from your `foo/3` or `foo/4`. It's just a nice convention.
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      1. USDaniel Lyons
 

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