On 2018-07-31 09:17, Shachar Shemesh wrote:
> I'm trying to figure out what's the signature of the built-in assert. It
> does not seem that I can define a similar function myself.
>
> First attempt:
> void myAssert(bool cond, string msg) @nogc nothrow;
>
> No, because msg gets evaluated unconditionally.
>
> void myAssert(bool cond, lazy string msg) @nogc nothrow;
>
> test.d(8): Error: @nogc function test.myAssert cannot call non-@nogc
> delegate msg
>
> void myAssert(bool cond, lazy string msg @nogc nothrow ) @nogc nothrow;
>
> test.d(4): Error: found @ when expecting )
> test.d(4): Error: semicolon expected following function declaration
> test.d(4): Error: no identifier for declarator nogc
> test.d(4): Error: declaration expected, not )
> test.d(9): Error: unrecognized declaration
>
> Templates to the rescue!!!
> void myAssert(STR)(bool cond, lazy STR msg );
>
> test.d(14): Error: @nogc function D main cannot call non-@nogc function
> test.myAssert!string.myAssert


Based on what Steven said and what I found in the spec, this seems to work with the current language [1]:

extern (C) int printf(in char*, ...) @nogc nothrow;

void myAssert(bool cond, string delegate() @nogc nothrow[1] msg ...) @nogc nothrow
{
    msg[0]();
}

string param() @nogc nothrow
{
    printf("bar\n");
    return "foo";
}

void main() @nogc nothrow
{
    myAssert(false, param());
}

This prints "bar". But if the delegate is not called, then nothing is printed.

The signature of "myAssert" is bit ugly and not straightforward to understand, but it works. I think it should be possible to specify attributes for lazy parameters or they should be automatically inferred. Is there an issue for this reported?

[1] https://run.dlang.io/is/g4Rm1w

-- 
/Jacob Carlborg

On Tuesday, 31 July 2018 at 20:03:39 UTC, ag0aep6g wrote:
> Looks like you can do it with a "lazy variadic function" [1],

Interesting, thanks. I've made a pull to add a See Also link about this to the Lazy Parameters docs:
https://github.com/dlang/dlang.org/pull/2439