I have simple test program:

import core.stdc.stdio : printf;

void test() {
    int* a;
    printf("a == null %d\n", a == null);
}

int function() fp = test;

extern (C) void main() {
    fp();
}

Why do I get:

\d\dmd-2.092.1\windows\bin64\dmd.exe -betterC tests.d
tests.d(5): Error: printf cannot be interpreted at compile time, because it has no available source code

This is on Windows

On Thursday, 19 November 2020 at 00:07:12 UTC, Dibyendu Majumdar wrote:
> int function() fp = test;

This tries to *call* the function test and assign its return value to fp.

You want &test to get the address.

On Thursday, 19 November 2020 at 00:08:59 UTC, Adam D. Ruppe wrote:
> On Thursday, 19 November 2020 at 00:07:12 UTC, Dibyendu Majumdar wrote:
>> int function() fp = test;
>
> This tries to *call* the function test and assign its return value to fp.
>

Really? why does it do that?

> You want &test to get the address.

On Thursday, 19 November 2020 at 00:08:59 UTC, Adam D. Ruppe wrote:
> On Thursday, 19 November 2020 at 00:07:12 UTC, Dibyendu Majumdar wrote:
>> int function() fp = test;
>
> You want &test to get the address.

Okay that works. Thanks

On 19/11/2020 1:11 PM, Dibyendu Majumdar wrote:
> On Thursday, 19 November 2020 at 00:08:59 UTC, Adam D. Ruppe wrote:
>> On Thursday, 19 November 2020 at 00:07:12 UTC, Dibyendu Majumdar wrote:
>>> int function() fp = test;
>>
>> This tries to *call* the function test and assign its return value to fp.
>>
> 
> Really? why does it do that?

You don't need the brackets to call a function (and with a little help from UFCS):

void main() {
	import std.stdio;
	
	"Hello!".writeln;
	writeln;
}

On Thursday, 19 November 2020 at 00:18:54 UTC, rikki cattermole wrote:

>
> You don't need the brackets to call a function (and with a little help from UFCS):
>
> void main() {
> 	import std.stdio;
> 	
> 	"Hello!".writeln;
> 	writeln;
> }

Okay thanks. Bad idea IMO.

On Thursday, 19 November 2020 at 00:20:50 UTC, Dibyendu Majumdar wrote:
> On Thursday, 19 November 2020 at 00:18:54 UTC, rikki cattermole wrote:
>
>>
>> You don't need the brackets to call a function (and with a little help from UFCS):
>>
>> void main() {
>> 	import std.stdio;
>> 	
>> 	"Hello!".writeln;
>> 	writeln;
>> }
>
> Okay thanks. Bad idea IMO.

Imagine what range pipelines would look like without it. This is one of my favorite D features.

On Thursday, 19 November 2020 at 00:20:50 UTC, Dibyendu Majumdar wrote:
> On Thursday, 19 November 2020 at 00:18:54 UTC, rikki cattermole
>>
>> You don't need the brackets to call a function (and with a little help from UFCS):
>>
>> void main() {
>> 	import std.stdio;
>> 	
>> 	"Hello!".writeln;
>> 	writeln;
>> }
>
> Okay thanks. Bad idea IMO.

Yes, calling `writeln` like that is a bad idea. That was a bad example.

But the actual reason is, this is how D implements properties [1]. Any function that doesn't take an argument can be called without parentheses. Any function which takes a single argument can be called like setting a field. Here's an example:

struct Color
{
    private uint hex;

    int red()
    out(result; result >= 0 && result <= 255) // assert that the result is within bounds
    {
        return (hex & 0xFF0000) >> 16;
    }

    void red(int value)
    in(value >= 0 && value <= 255) // assert that the value is within bounds
    {
        hex = (hex & 0x00FFFF) | (value << 16);
    }

    // similar functions for green and blue
}

void main()
{
    Color color;
    color.red = 255;
    assert(color.red == 255);
}

[1] https://en.wikipedia.org/wiki/Property_(programming)

--
/Jacob Carlborg

On Thursday, 19 November 2020 at 01:42:16 UTC, Mike Parker wrote:
> On Thursday, 19 November 2020 at 00:20:50 UTC, Dibyendu Majumdar wrote:
>> On Thursday, 19 November 2020 at 00:18:54 UTC, rikki cattermole wrote:
>>
>>>
>>> You don't need the brackets to call a function (and with a little help from UFCS):
>>>
>>> void main() {
>>> 	import std.stdio;
>>> 	
>>> 	"Hello!".writeln;
>>> 	writeln;
>>> }
>>
>> Okay thanks. Bad idea IMO.
>
> Imagine what range pipelines would look like without it. This is one of my favorite D features.

Well Java and C# have streams and it looks perfectly fine without this kind of syntax.

On Thursday, 19 November 2020 at 09:23:25 UTC, Jacob Carlborg wrote:
> Yes, calling `writeln` like that is a bad idea. That was a bad example.
>
> But the actual reason is, this is how D implements properties [1]. Any function that doesn't take an argument can be called without parentheses. Any function which takes a single argument can be called like setting a field.

I think that properties on an object are a special case - but treating an random function identifier as callable is still bad.