• "Michael" <pr m1xa.com> Jan 02 2013
• "Maxim Fomin" <maxim maxim-fomin.ru> Jan 02 2013
• Timon Gehr <timon.gehr gmx.ch> Jan 03 2013
• "Timon Gehr" <timon.gehr gmx.ch> Jan 02 2013
• "Michael" <pr m1xa.com> Jan 03 2013
• "Michael" <pr m1xa.com> Jan 03 2013

"Michael" <pr m1xa.com> writes:

R  With(I, R)(I o, R function (I) fun)
{
	static if(isAssignable!(I, typeof(null)))
		return o is null ? null : fun(o);
	else
		return fun(o);
}

class Person
{
	private
	{
		string  _name;
		Address _address;
	}

	 property
	{
		string name() { return	_name; }
		void   name(string v) { _name = v; }

		Address address() { return _address; }
		void	address(Address v) { _address = v; }
	}
}

in main function
----------------
foreach(p; persons)	
	p.With((Person x) => x.address); // works

but
----------------
foreach(p; persons)	
		p.With(x => x.address); // error

nullcheck.d(89): Error: template maybe.With does not match any 
function template
  declaration. Candidates are:
maybe.d(20):        maybe.With(I, R)(I o, R function(I) fun)
nullcheck.d(89): Error: template maybe.With(I, R)(I o, R 
function(I) fun) cannot
  deduce template function from argument types !()(Person,void)


Why?

"Maxim Fomin" <maxim maxim-fomin.ru> writes:

On Wednesday, 2 January 2013 at 21:00:10 UTC, Michael wrote:
 R  With(I, R)(I o, R function (I) fun)
 {
 	static if(isAssignable!(I, typeof(null)))
 		return o is null ? null : fun(o);
 	else
 		return fun(o);
 }

 class Person
 {
 	private
 	{
 		string  _name;
 		Address _address;
 	}

 	 property
 	{
 		string name() { return	_name; }
 		void   name(string v) { _name = v; }

 		Address address() { return _address; }
 		void	address(Address v) { _address = v; }
 	}
 }

 in main function
 ----------------
 foreach(p; persons)	
 	p.With((Person x) => x.address); // works

 but
 ----------------
 foreach(p; persons)	
 		p.With(x => x.address); // error

 nullcheck.d(89): Error: template maybe.With does not match any 
 function template
  declaration. Candidates are:
 maybe.d(20):        maybe.With(I, R)(I o, R function(I) fun)
 nullcheck.d(89): Error: template maybe.With(I, R)(I o, R 
 function(I) fun) cannot
  deduce template function from argument types !()(Person,void)


 Why?


The first one is a lambda function, the second one is a lambda 
template. Templates have type void.

Timon Gehr <timon.gehr gmx.ch> writes:

On 01/03/2013 10:54 PM, Michael wrote:
 Thanks guys)

 auto With(alias fun, I)(I o) // maybe add a template constraint here
 {
     static if(isAssignable!(I, typeof(null)))
         return o is null ? null : fun(o);
     else
         return fun(o);
 }

 foreach(p; persons)
     p.With!(x => x.address);


 Now if I want add a somewhat task into expression "x => x.address", for
 example "writeln(x.address)", a code should be rewritten like

 foreach(p; persons)
     p.With!(x => {writeln(x.address); return x.address;}());


 As I understand, right part of expression above - "{ ... }()" is
 anonymous function (or delegate, or closure) that immediately called in
 lambda expression. Right? Is right behaviour?


You understand this correctly, but it is a somewhat roundabout way to 
achieve what you want to do.

This should work as well:

foreach(p; persons)
     p.With!((x){writeln(x.address); return x.address;});

The reason is that there are two different ways for forming function 
literals. The lambda literal a=>exp is immediately rewritten to (a){ 
return exp; }

Historically, the lambda syntax has not been available, turning function 
literal heavy code into something that looked like
{return{return(){return((){return}())}. The introduction of '=>' was a 
backwards-compatible addition.

"Timon Gehr" <timon.gehr gmx.ch> writes:

On Wednesday, 2 January 2013 at 21:12:33 UTC, Maxim Fomin wrote:
 On Wednesday, 2 January 2013 at 21:00:10 UTC, Michael wrote:
 R  With(I, R)(I o, R function (I) fun)
 {
 	static if(isAssignable!(I, typeof(null)))
 		return o is null ? null : fun(o);
 	else
 		return fun(o);
 }

 ...

 in main function
 ----------------
 foreach(p; persons)	
 	p.With((Person x) => x.address); // works

 but
 ----------------
 foreach(p; persons)	
 		p.With(x => x.address); // error

 nullcheck.d(89): Error: template maybe.With does not match any 
 function template
 declaration. Candidates are:
 maybe.d(20):        maybe.With(I, R)(I o, R function(I) fun)
 nullcheck.d(89): Error: template maybe.With(I, R)(I o, R 
 function(I) fun) cannot
 deduce template function from argument types !()(Person,void)


 Why?


 The first one is a lambda function, the second one is a lambda 
 template. Templates have type void.


Actually the 'void' is just a diagnostics bug. (lambda templates 
only exist in template parameter lists.) The reason the matching 
fails is that IFTI is not smart enough to devise a type for the 
parameter 'x', and therefore no type for 'R' is obtained, what 
makes the matching fail. Currently other parameters are not taken 
into account during IFTI matching. I believe that the reason is 
that otherwise the compiler needs to be clever about the order in 
which it analyzes the parameters. I consider this one of the more 
annoying limitations.

A workaround that should work in this case is to use a template 
parameter:

auto With(alias fun, I)(I o) // maybe add a template constraint 
here
{
	static if(isAssignable!(I, typeof(null)))
		return o is null ? null : fun(o);
	else
		return fun(o);
}

---

foreach(p; persons)	
     p.With!(x => x.address);

This creates a template from the lambda, and instantiates it once 
inside 'With'. This way the return type does not have to be part 
of the template parameters.

"Michael" <pr m1xa.com> writes:

Thanks guys)

 auto With(alias fun, I)(I o) // maybe add a template constraint 
 here
 {
 	static if(isAssignable!(I, typeof(null)))
 		return o is null ? null : fun(o);
 	else
 		return fun(o);
 }

 foreach(p; persons)	
     p.With!(x => x.address);


Now if I want add a somewhat task into expression "x => 
x.address", for example "writeln(x.address)", a code should be 
rewritten like

 foreach(p; persons)
     p.With!(x => {writeln(x.address); return x.address;}());


As I understand, right part of expression above - "{ ... }()" is 
anonymous function (or delegate, or closure) that immediately 
called in lambda expression. Right? Is right behaviour?

"Michael" <pr m1xa.com> writes:

I just doing a chained null checks.

And I prefer when code looks like "a => ...".

In any case we have two ways to do same thing: "a => {...}()" and 
"(a){...}".

Thanks Timon)