Re: Delegate function access to classes local variable

[Colin Grogan]

On Friday, 8 November 2013 at 13:19:05 UTC, Colin Grogan wrote:
On Friday, 8 November 2013 at 13:14:33 UTC, Timon Gehr wrote:
On 11/08/2013 01:43 PM, Colin Grogan wrote:
Hi folks,
I'm having some issue getting a delegate function access to a 
classes
member variable.

At object construct time, I'm passing in a delegate function, 
and a list
of parameters after.
The parameters are saved to a variable called vars.
Should I then not be able to access that vars variable from 
inside my
delegate function?

I guess some code is a better explanation:
import std.stdio;
void main()
{
   Column!string col1 = new Column!string( {return "test"; }, 
"Hello, ");
   Column!string col2 = new Column!string( {return vars[0]; },
"World"); // Compilation fail!! Delegate cant see vars[0]

It is not even in scope here.

   writef("%s", col1.nextValue);
   writefln("%s", col2.nextValue);
// I want it to print "Hello, World" here
}

public class Column(Vars...){
   private Vars vars;
   public string delegate() func;

   public this(string delegate() func, Vars vars){
       this.vars = vars;
       this.func = func;
   }

   public string nextValue(){
       return this.func();
   }
}


The compilation error is:
source/app.d(5): Error: undefined identifier vars

This has been wrecking my head for a couple days now, I'm 
half way
resigned to the fact it cant work but said I'd ask here to be 
sure.

Thanks!

import std.stdio;
void main(){
   Column!string col1 = new Column!string((ref m)=>"Hello, ", 
"test");
   Column!string col2 = new Column!string((ref m)=>m.vars[0], 
"World");
   writef("%s", col1.nextValue);
   writefln("%s", col2.nextValue);
}

public class Column(Vars...){
   struct Members{ Vars vars; }
   private Members members;
   alias members this;
   string delegate(ref Members) func;

   this(string delegate(ref Members) func, Vars vars){
       this.vars = vars;
       this.func = func;
   }

   string nextValue(){
       return func(members);
   }
}


Ah, brilliant! I like that construct.
Thank you!

My optimism may have been premature.
After trying this out today, I havent been able to pass in 
anything more complex than a 1 line to the constructor.

For example,
    Column!(int, int) randonNumberColumn = new Column!(int, 
int)((ref m)=>to!string(uniform(m.vars[0], m.vars[1])), 1, 10);

will work.
However,
    Column!(int, int) incrementalNumberColumn = new Column!(int, 
int)((ref m)=>{m.vars[0]+=m.vars[1]; return 
to!string(m.vars[0]-m.vars[1]);}, 1,2);

wont.

Maybe my syntax is just wrong or this is simply a limitation?

Also, if you could explain what the => operator is doing there 
that would be great. I couldnt find the info on it in the docs...