We need a way to make functions pure and/or nothrow based on the

[bearophile]

Jonathan M Davis:

>The one problem I see is that if the compiler has to determine whether a given function can be pure and/or nothrow, it's going to potentially have to go arbitrarily deep into the call hierarchy to figure it out<

This is already done for pure functions, const, immutable, nothrow, etc, all of them are transitive.


> Does anyone have some good suggestions on how to solve this issue?

I have an enhancement request on it, of course:
http://d.puremagic.com/issues/show_bug.cgi?id=5125

There I have suggested a @optional_tag(), the first argument is a compile-time boolean and the second is an attribute/keyword like pure, nothrow, @safe, etc.:


@optional_tag(isPure!F, pure) int[] map(F)(F f, int[] data) {
    int[] res;
    foreach (x; data)
        res ~= f(x);
    return res;
}


Where isPure is just:

import std.traits: FunctionAttribute, functionAttributes;

template isPure(F) {
    enum bool isPure = functionAttributes!(F) & FunctionAttribute.PURE;
}


The name "optional_tag" may be improved. This solution is not much general, so you may invent something more elegant.

Bye,
bearophile