Re: Lambda capture by value

[Steven Schveighoffer via Digitalmars-d-learn]

On 2/24/20 3:32 PM, H. S. Teoh wrote:


To fix this, copy the value of 'i' to a local variable inside the loop
body, then the lambda will correctly capture a unique per-iteration
instance of the variable.  Like this:

 foreach (i; iota(5))
 {
auto _i = i;
 printers[i] = () { write(_i); };
 }


Nope. It doesn't ;) Because actually, _i is reused for the loop iteration.

You are thinking that the capture happens on the delegate creation. In 
essence, it just sets a flag to the compiler that the enclosing function 
must be placed on the heap.


Adam's method works because the nested call's stack frame is captured 
when you return that delegate.


In other words, it looks like this:

void main()
{
   static struct __mainStackFrame
   {
   int i;
   int _i;
   }

   __mainStackFrame *frame = new __mainStackFrame; // here's where the 
capture happens

   with(*frame)
   {
  // main's function body, with all variables already being declared
   }
}

-Steve

Re: Lambda capture by value

[Elronnd via Digitalmars-d-learn]

printers[i] = () { write(i); };


I know it looks silly but if you make that:

 printers[i] = (int i) { return () { write(i); }; }(i);

it will do what you want.


Or, the slightly prettier (imo)

  printers[i] = ((i) => () => write(i))(i);

Or, if you need to force it to return void:

  printers[i] = ((i) => {write(i);})(i);

Re: Lambda capture by value

[H. S. Teoh via Digitalmars-d-learn]

On Mon, Feb 24, 2020 at 07:50:23PM +, JN via Digitalmars-d-learn wrote:
> import std.range;
> import std.stdio;
> 
> alias NumberPrinter = void delegate();
> 
> NumberPrinter[int] printers;
> 
> void main()
> {
> foreach (i; iota(5))
> {
> printers[i] = () { write(i); };
> }
> 
> foreach (i; iota(5))
> {
> printers[i]();
> }
> }
> 
> This prints 4 4 4 4 4.
> 
> How to make it so that it prints 0 1 2 3 4? Is it possible without
> changing the delegate definition to void delegate(int)?

The cause of the problem is that 'i' in the first foreach loop is
*reused* across loop iterations, so all 5 lambdas are actually closing
over the same variable, which gets its value replaced by the next
iteration.

To fix this, copy the value of 'i' to a local variable inside the loop
body, then the lambda will correctly capture a unique per-iteration
instance of the variable.  Like this:

foreach (i; iota(5))
{
auto _i = i;
printers[i] = () { write(_i); };
}
 

T

-- 
Curiosity kills the cat. Moral: don't be the cat.

Re: Lambda capture by value

[kinke via Digitalmars-d-learn]

On Monday, 24 February 2020 at 19:50:23 UTC, JN wrote:

foreach (i; iota(5))
{
printers[i] = () { write(i); };
}


This allocates 1 closure and generates 1 lambda, so all printers 
are identical delegates. You could use a static foreach:


NumberPrinter[] printers;
static foreach (i; 0..5)
printers ~= () { write(i); };
foreach (d; printers)
d();

Re: Lambda capture by value

[JN via Digitalmars-d-learn]

On Monday, 24 February 2020 at 20:00:20 UTC, Adam D. Ruppe wrote:

On Monday, 24 February 2020 at 19:50:23 UTC, JN wrote:

foreach (i; iota(5))
{
printers[i] = () { write(i); };


I know it looks silly but if you make that:

 printers[i] = (int i) { return () { write(i); }; }(i);

it will do what you want.

This is something that used to be common in javascript, write a 
little function that passes the capture-by-value args and 
returns the lambda you actually want and call it immediately.


That extra layer causes the compiler to create a new copy of 
the capture variables snapshotted in time.


D'oh! I am actually familiar with the pattern from Javascript, 
used it many times, but somehow got it mixed up with something 
else and couldn't make it work.


Thanks.

Re: Lambda capture by value

[Adam D. Ruppe via Digitalmars-d-learn]

On Monday, 24 February 2020 at 19:50:23 UTC, JN wrote:

foreach (i; iota(5))
{
printers[i] = () { write(i); };


I know it looks silly but if you make that:

 printers[i] = (int i) { return () { write(i); }; }(i);

it will do what you want.

This is something that used to be common in javascript, write a 
little function that passes the capture-by-value args and returns 
the lambda you actually want and call it immediately.


That extra layer causes the compiler to create a new copy of the 
capture variables snapshotted in time.