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digitalmars.D.learn - How to hand in a closure variable

reply "Bienlein" <jeti789 web.de> writes:
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
Mar 24 2014
next sibling parent "Dicebot" <public dicebot.lv> writes:
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;
Mar 24 2014
prev sibling parent reply Matej Nanut <matejnanut gmail.com> writes:
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.
Mar 24 2014
parent reply "Bienlein" <jeti789 web.de> writes:
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.
Apr 04 2014
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
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
Apr 04 2014
parent reply "Bienlein" <jeti789 web.de> writes:
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.
Apr 04 2014
next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
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
Apr 04 2014
parent "Steve Teale" <steve.teale britseyeview.com> writes:
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
Apr 07 2014
prev sibling parent reply "Jesse Phillips" <Jesse.K.Phillips+D gmail.com> writes:
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; 
}}();
Apr 04 2014
parent "Bienlein" <jeti789 web.de> writes:
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;
     }
}();
Apr 07 2014