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digitalmars.D.learn - Compile time only delegates

reply "Frustrated" <Frustrated nowhere.com> writes:
Due to a previous issue I am trying to do the following

mixin template A()
{
     mixin((function () => "int x;")() );
}

the problem is that the compiler will not let me use the delegate 
because it has no this context. Of course the whole point here is 
that the delegate will never be used at runtime since it is used 
in the mixin(which is purely compile time).

It should work(there is no reason it shouldn't) yet it doesn't.

The goal is that I do not want to create a function outside the 
mixin to use because it will be included in the context of the 
template mixin. e.g.,

mixin template A()
{
     string B() { return "int x;"; }
     mixin(B()); // still doesn't work but if it did B would be 
inserted in the context of the template. e.g., if used in a class 
the class would then have a method named B in it, hence the use 
of the lambda.
}
Mar 25 2014
parent reply "Meta" <jared771 gmail.com> writes:
On Tuesday, 25 March 2014 at 18:05:47 UTC, Frustrated wrote:

 Due to a previous issue I am trying to do the following

 mixin template A()
 {
     mixin((function () => "int x;")() );
 }

 the problem is that the compiler will not let me use the 
 delegate because it has no this context. Of course the whole 
 point here is that the delegate will never be used at runtime 
 since it is used in the mixin(which is purely compile time).

 It should work(there is no reason it shouldn't) yet it doesn't.

 The goal is that I do not want to create a function outside the 
 mixin to use because it will be included in the context of the 
 template mixin. e.g.,

 mixin template A()
 {
     string B() { return "int x;"; }
     mixin(B()); // still doesn't work but if it did B would be 
 inserted in the context of the template. e.g., if used in a 
 class the class would then have a method named B in it, hence 
 the use of the lambda.
 }


There shouldn't be a context pointer at all as you specified that 
it's a function, not a delegate. Function literals don't have 
context pointers as far as I know. Did you try:

mixin((function () { return "int x;" })());

Just to be sure?
Mar 25 2014
next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
This works for me (2.064):

import std.stdio : writeln;

void main() {
  mixin({ return "int x;"; }());
  writeln(x);
}
Mar 25 2014
prev sibling parent reply "Frustrated" <Frustrated nowhere.com> writes:
On Tuesday, 25 March 2014 at 18:10:17 UTC, Meta wrote:

 On Tuesday, 25 March 2014 at 18:05:47 UTC, Frustrated wrote:

 Due to a previous issue I am trying to do the following

 mixin template A()
 {
    mixin((function () => "int x;")() );
 }

 the problem is that the compiler will not let me use the 
 delegate because it has no this context. Of course the whole 
 point here is that the delegate will never be used at runtime 
 since it is used in the mixin(which is purely compile time).

 It should work(there is no reason it shouldn't) yet it doesn't.

 The goal is that I do not want to create a function outside 
 the mixin to use because it will be included in the context of 
 the template mixin. e.g.,

 mixin template A()
 {
    string B() { return "int x;"; }
    mixin(B()); // still doesn't work but if it did B would be 
 inserted in the context of the template. e.g., if used in a 
 class the class would then have a method named B in it, hence 
 the use of the lambda.
 }


 There shouldn't be a context pointer at all as you specified 
 that it's a function, not a delegate. Function literals don't 
 have context pointers as far as I know. Did you try:

 mixin((function () { return "int x;" })());

 Just to be sure?


Thanks, I thought I did try that(that was the first thing I tried 
I thought). Seems to be working though.
Mar 25 2014
parent "Frustrated" <Frustrated nowhere.com> writes:
On Tuesday, 25 March 2014 at 20:20:29 UTC, Frustrated wrote:

 On Tuesday, 25 March 2014 at 18:10:17 UTC, Meta wrote:

 On Tuesday, 25 March 2014 at 18:05:47 UTC, Frustrated wrote:

 Due to a previous issue I am trying to do the following

 mixin template A()
 {
   mixin((function () => "int x;")() );
 }

 the problem is that the compiler will not let me use the 
 delegate because it has no this context. Of course the whole 
 point here is that the delegate will never be used at runtime 
 since it is used in the mixin(which is purely compile time).

 It should work(there is no reason it shouldn't) yet it 
 doesn't.

 The goal is that I do not want to create a function outside 
 the mixin to use because it will be included in the context 
 of the template mixin. e.g.,

 mixin template A()
 {
   string B() { return "int x;"; }
   mixin(B()); // still doesn't work but if it did B would be 
 inserted in the context of the template. e.g., if used in a 
 class the class would then have a method named B in it, hence 
 the use of the lambda.
 }


 There shouldn't be a context pointer at all as you specified 
 that it's a function, not a delegate. Function literals don't 
 have context pointers as far as I know. Did you try:

 mixin((function () { return "int x;" })());

 Just to be sure?


 Thanks, I thought I did try that(that was the first thing I 
 tried I thought). Seems to be working though.


oops, sorry, same problem. When used inside a class says I need 
this:

Heres the actual code:



import std.stdio, std.cstream;

// mixin template used to wrap the string mixin
mixin template C()
{
	mixin((function () { return "void foo(iA a) { 
writeln(`Generic`);  }"; })());
}

// eponymously named template used as a string mixin to generate 
the function
template B()
{
	string B() { return "void foo(iA a) { writeln(`Generic`);  }"; }
}

interface iA {	void foo(iA a); }

class A : iA
{
	void foo(A a) { writeln("Specific"); }
	//mixin(B);
	mixin C;    // doesn't work, use line above (mixin templates 
won't override even if different parameters)
}


void main()
{
	
	iA a = new A;
	a.foo(a);					// Generic/Generic
	(cast(A)a).foo(cast(A)a);	// Specific/Specific
	a.foo(cast(A)a);			// Generic/Specific
	(cast(A)a).foo(a);			// Specific/Generic     line doesn't work 
when using mixin template
	
	writeln("-----");
	mixin C;
	foo(a);

	din.getc();
}



and the error:

Error: function main.A.C!().__funcliteral1 need 'this' to access 
member __funcliteral1	

(this only happens using the mixin C in the class, not in main or 
when using mixin B)
Mar 25 2014