Continuations in D?

[David Medlock]

Unknown W. Brackets wrote:

> Please forgive my ignorance; what is a continuation or where can I read 
> about it?
> 
> Thanks,
> -[Unknown]
> 
> 
>> Does D support continuations?  I anticipate that continuations will 
>> become
>> increasingly important as people seek ways to retain the simplicity of 
>> a threaded architecture while
>> avoiding the overhead of actual threads.  If D could support this 
>> language feature, I think it would make D very
>> appealing for a whole variety of applications.
>>
>> Ian.
>>
>>

To my knowledge of them, they are an alternate form of writing code.

Consider a function which returns an integer:

int foo() { return 1; }

Now suppose you wanted to return multiple integers:

int foo() { return 2; return 3; return 4; } // doesnt work

to do this requires an inversion of control, the called function will 
call you back with the values:

void foo( void delegate( int ) yield )
{
   yield( 2 );
   yield( 3 );
   yield( 4 );
}

This is commonly known as continuation passing style or CPS.  In this 
style the function passed to a function represents the 'rest of the 
program'.  In this way the function can complete the program more than 
one, hence a continuation of the current program.

Converting the code to this form allows you to use pieces of code in 
various ways.  Consider try/catch.

try
{
   calc1();
   calc2();
}
catch( ... ) { ..various fail code... }

Using this you can see the conversion of this to CPS form:

void failure_code() { ..various fail code... }

void try_function( void delegate() failure )
{
   if ( calc1 == FAIL ) failure();
   else if ( calc2 == FAIL ) failure();
}

try_function( &failure_code )

Inner functions in D make this style of code very workable.

PS. Note that with an appropriate return value from failure() you can 
signify that the failed calculation be retried!

This is a simplification but if you google there are several good papers 
on them.

-DavidM