Elvis operator - shortened ternary operator (page 2) - D Programming Language Discussion Forum

6 days ago

Re: Elvis operator - shortened ternary operator

Posted by Quirin Schroll
in reply to Paul Backus

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Quirin Schroll

Posted in reply to Paul Backus

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On Friday, 28 March 2025 at 17:06:56 UTC, Paul Backus wrote:

On Thursday, 27 March 2025 at 00:26:27 UTC, Andrey Zherikov wrote:

See wiki for details.

Proposed syntax:

A ?: B

This is equivalent to:

auto R = A;  // temporary (hidden) variable
(cast(bool) R) ? R : B;

Semantics: return A if it's truthy, or B otherwise.

Benefit: avoids evaluating if A twice which might be a call to some function with side effects.

Library version:

auto orElse(T)(T a, lazy T b)
{
    return a ? a : b;
}

unittest
{
    string s1 = "hello";
    string s2 = null;

    assert(s1.orElse("goodbye") == "hello");
    assert(s2.orElse("goodbye") == "goodbye");
}

That orElse cannot preserve value categories. Adding auto ref to ´lazy T b` isn’t something the language supports. I’m not sure why, maybe it’s merely an oversight, maybe it’s because it’s complicated.

auto ref orElse(T, DG)(auto ref T a, DG b)
{
    return a ? a : b();
}

void test()
{
    int x;
    int y;
    x.orElse(auto ref () => y)  = 10;
    assert(y == 10);

    x = 1;
    x.orElse(auto ref () => y)  = 11;
    assert(x == 11 && y == 10);
}

It works, but it’s not really great.

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