January 05, 2012
import std.stdio;

void test(T)(lazy T dg)
{
    test2(dg);
}

void test2(T)(lazy T dg)
{
    dg();    // nothing happens
    dg()();  // have to use double-invocation instead
}

void main()
{
    test({ writeln("test"); });
}

Do you think it would be possible for the compiler to avoid wrapping delegates into another delegate? I'm having this problem with this sort of template:

import std.conv;
import std.string;

auto onFailThrow(E, T)(lazy T dg)
{
    try
    {
        static if (is(ReturnType!T == void))
            dg();
        else
            return dg();
    }
    catch (Exception ex)
    {
        throw new E(ex.msg);
    }
}

void main()
{
    string x = "x";
    string y = "y";
    onFailThrow!Exception({ to!int(x); });
    onFailThrow!Exception(to!int(y));
}

The first call doesn't do anything because the delegate is wrapped inside of another delegate. I want this template to be versatile enough to be used by both lazy expressions and delegate literals, but I don't know how.

If I write the same template but with "lazy" stripped off I'll have conflicting templates, but I don't know how I would write constraints to separate the two. Any ideas?
January 07, 2012
On 5/01/12 5:26 AM, Andrej Mitrovic wrote:
> The first call doesn't do anything because the delegate is wrapped
> inside of another delegate. I want this template to be versatile
> enough to be used by both lazy expressions and delegate literals, but
> I don't know how.

void test(T)(lazy T dg)
{
    static if (is(T == delegate))
        dg()();
    else
        dg();
}

void main()
{
    test( writeln("a") );
    test( { writeln("b"); } );
}
January 07, 2012
Hah, I never thought of using that check. Thanks.
January 09, 2012
On 05/01/2012 05:26, Andrej Mitrovic wrote:
<snip>
> The first call doesn't do anything because the delegate is wrapped
> inside of another delegate. I want this template to be versatile
> enough to be used by both lazy expressions and delegate literals, but
> I don't know how.
<snip>

If you have a delegate you want to use as a lazy expression, you can make the lazy argument a call to it

    onFailThrow!Exception({ to!int(x); }());

Of course, Peter's solution enables you to omit the () and just pass the delegate in, but it does mean that you can't lazily evaluate an expression to a delegate, unless you wrap it in a delegate literal.

Stewart.
Top | Discussion index | About this forum | D home