Hello,

I have some piece of code that compiles and runs fine:

void main(string[] args)
{	
	int a = 7;
	int delegate() dg = { return a + 3; };
	auto result = dg();
	writeln(result);
}

Now I want the closure (aka delegate) to have a closure variable:

int a = 7;
int delegate(int) dg = { value => return value + a + 3; };
auto result = dg(123);

Unhappily, the code above doesn't compile. Tried various things, looked for samples on the D hompepage and in the book by Çehreli, but had no luck.

Some hints appreciated.
Thanks, Bienlein

On Monday, 24 March 2014 at 16:40:55 UTC, Bienlein wrote:
> Now I want the closure (aka delegate) to have a closure variable:
>
> int a = 7;
> int delegate(int) dg = { value => return value + a + 3; };
> auto result = dg(123);
>
> Unhappily, the code above doesn't compile. Tried various things, looked for samples on the D hompepage and in the book by Çehreli, but had no luck.
>
> Some hints appreciated.
> Thanks, Bienlein

auto dg = (int value) { return value + a + 3; };

or short-hand form:

auto dg = (int value) => value + a + 3;

Hello!

You just missed the syntax a little.

Instead of:

> int delegate(int) dg = { value => return value + a + 3; };

You can write

    auto dg = (int value) { return value + a + 3; }; // Omitted return
type, but had to specify type of value.

or

    auto dg = (int value) => value + a + 3; // Notice no "return" keyword.

or

    int delegate(int) dg = value => value + a + 3; // Omitted type of
value, but had to write the full type of dg.

You can also write a delegate as an inner function:

    int a = 7;
    int dg(int value)
    {
        return value + a + 3;
    }
    auto result = dg(123);

I'm not sure, but I guess all of these should mean the same thing.

Thanks so far. I have another one, though. Not trying to tease people, I really don't know ;-).

This compiles and runs:

immutable int a = (int val) {
	if(1 == 1) {
		return val;
	} else {
		return 456;
	}
}(123);	
	
writeln(a);

Whereas this does not compile:

immutable int b = (int) {
	if(1 == 1) {
		return 123;
	} else {
		return 456;
	}
}(); // line x	

However, this does compile and displays correctly 123:

immutable int b = (int) {
	if(1 == 1) {
		return 123;
	} else {
		return 456;
	}
}(1); // line y	
		
writeln(b);

Note it says () in line x and (1) in line y. The (1) in line y is redundant, but the stuff then compiles.

Bienlein:

> Whereas this does not compile:
>
> immutable int b = (int) {
> 	if(1 == 1) {
> 		return 123;
> 	} else {
> 		return 456;
> 	}
> }(); // line x	
>
> However, this does compile and displays correctly 123:
>
> immutable int b = (int) {
> 	if(1 == 1) {
> 		return 123;
> 	} else {
> 		return 456;
> 	}
> }(1); // line y	
> 		
> writeln(b);
>
> Note it says () in line x and (1) in line y. The (1) in line y is redundant, but the stuff then compiles.

If your D function has one argument, you have to give it one argument, even if it doesn't have a visible name and it's unused.

Bye,
bearophile

On Friday, 4 April 2014 at 13:53:33 UTC, bearophile wrote:

> If your D function has one argument, you have to give it one argument, even if it doesn't have a visible name and it's unused.

Ah! Admittedly, I though it's the return type .. So this works
now:

immutable int b = () {
	if(1 == 1) {
		return 123;
	} else {
		return 456;
	}
}();

What I was actually looking for was how to get this to work:

immutable int b = if(1 == 1) { return 123; } else { return 456; };

But I'm happy enough with the solution through a delegate.

Bienlein:

> What I was actually looking for was how to get this to work:
>
> immutable int b = if(1 == 1) { return 123; } else { return 456; };

immutable b = (1 == 1) ? 123 : 456;

Bye,
bearophile

On Friday, 4 April 2014 at 15:13:25 UTC, Bienlein wrote:
> What I was actually looking for was how to get this to work:
>
> immutable int b = if(1 == 1) { return 123; } else { return 456; };
>
> But I'm happy enough with the solution through a delegate.

What bearophile said, or:

immutable int b = {if(1 == 1) { return 123; } else { return 456; }}();

On Friday, 4 April 2014 at 19:56:14 UTC, Jesse Phillips wrote:
> On Friday, 4 April 2014 at 15:13:25 UTC, Bienlein wrote:
>> What I was actually looking for was how to get this to work:
>>
>> immutable int b = if(1 == 1) { return 123; } else { return 456; };
>>
>> But I'm happy enough with the solution through a delegate.
>
> What bearophile said, or:
>
> immutable int b = {if(1 == 1) { return 123; } else { return 456; }}();

Thanks, that's it! Now I can also do what I initialliy wanted to (e.g. have several lines of code in the expression blocks):

immutable int b = {
    if(1 == 1) {
        writeln("123");
        return 123;
    } else {
        writeln("456");
        return 456;
    }
}();

On Friday, 4 April 2014 at 15:15:55 UTC, bearophile wrote:
> Bienlein:
>
>> What I was actually looking for was how to get this to work:
>>
>> immutable int b = if(1 == 1) { return 123; } else { return 456; };
>
> immutable b = (1 == 1) ? 123 : 456;
>
> Bye,
> bearophile

You said you did not like ternary expressions ;=)

Steve