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digitalmars.D - repost: semantics of the keyword lazy

reply Karen Lanrap <karen digitaldaemon.com> writes:
I have posted the following in the learn-group without any answers:


From the documentation:

Lazy arguments are evaluated not when the function is called, but 
when the parameter is evaluated within the function.


First to note, that this is a circular definition without any fixed 
point to start from:
a lazy argument is evaluated, when it is evaluated :-(


Maybe Walter meant something like this:
With the exception of lazy parameters all actual parameters are 
passed evaluated to the function they belong to.


But this does not help for lazy parameters: when the hell are they 
evaluated?

"Within the function" Walters says. But what does this mean if the 
calling function is recursive or combined with parameters that are 
not lazy evaluated? Example:

int ack( lazy bool b, int x, lazy int y){
    if( x == 0 )
        if( b) return y+1;
    else
        if( y == 0)
            return ack( b, x-1, 1);
        else
            return ack( b, x-1, ack( b, x, y-1));
}

If Walter wanted to express, that lazy parameters must be evaluated 
in the calling function, then I understand, but I doubt, that this 
concept is that useful.

If on the other hand lazy parameters can be passed down through 
function hierarchies one should be able to protect them from 
accidental evaluation until they reach the target point of their 
evaluation, i.e. something like

      if( unlazy b) return y+1;

should be possible and
 
      ack( x, b, y-1)

should show up as an error because the target is not notified as

      ack( x, unlazy b, y-1)
Oct 11 2006
next sibling parent reply Derek Parnell <derek nomail.afraid.org> writes:
On Thu, 12 Oct 2006 03:08:07 +0000 (UTC), Karen Lanrap wrote:


 I have posted the following in the learn-group without any answers:
 
 From the documentation:
 
 Lazy arguments are evaluated not when the function is called, but 
 when the parameter is evaluated within the function.
 
 First to note, that this is a circular definition without any fixed 
 point to start from:
 a lazy argument is evaluated, when it is evaluated :-(


I have not read that way at all. To me this is not a circular definition. 

Non-lazy arguments are evaluated by the code that initiates the call to the
function. Lazy ones are evaluated by code that lies inside the function;
that is they are evaluated after the function is called, by your code in
the function.


 Maybe Walter meant something like this:
 With the exception of lazy parameters all actual parameters are 
 passed evaluated to the function they belong to.


That is exactly what he meant and said.
 

 But this does not help for lazy parameters: when the hell are they 
 evaluated?


Whenever the called function decides to do it, if at all. To evaluate a
lazy argument, the function must explicitly call the delegate (which the
lazy argument represents).


 "Within the function" Walters says. But what does this mean if the 
 calling function is recursive or combined with parameters that are 
 not lazy evaluated? Example:
 
 int ack( lazy bool b, int x, lazy int y){
     if( x == 0 )
         if( b) return y+1;
     else
         if( y == 0)
             return ack( b, x-1, 1);
         else
             return ack( b, x-1, ack( b, x, y-1));
 }
 
 If Walter wanted to express, that lazy parameters must be evaluated 
 in the calling function, then I understand, but I doubt, that this 
 concept is that useful.


But that is exactly what they are. It just means that the function
evaluates them by treating them as delegates and calling them, using the
result as the evaluated value.
 

 If on the other hand lazy parameters can be passed down through 
 function hierarchies one should be able to protect them from 
 accidental evaluation until they reach the target point of their 
 evaluation


'accidental'? They are passed down as delegates and only evaluated if
explicitly called. No accident.


-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
12/10/2006 1:49:14 PM
Oct 11 2006
next sibling parent reply Hasan Aljudy <hasan.aljudy gmail.com> writes:
I was wondering what would happen if you call the lazy "delegate" 
multiple times. will the result be cached or will the delegate/function 
be called multiple times?
Oct 11 2006
parent Derek Parnell <derek nomail.afraid.org> writes:
On Wed, 11 Oct 2006 22:56:59 -0600, Hasan Aljudy wrote:


 I was wondering what would happen if you call the lazy "delegate" 
 multiple times. will the result be cached or will the delegate/function 
 be called multiple times?


It is called each time and the values are not cached.

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
12/10/2006 5:05:00 PM
Oct 12 2006
prev sibling parent Karen Lanrap <karen digitaldaemon.com> writes:
Derek Parnell wrote:


 Whenever the called function decides to do it, if at all.


Hopefully the programmer has full control over what the called 
function decides :-(


Another try:

The evaluation of actual parameters can be delayed to any level in 
the call hierarchy.

If the evaluation of an actual parameter has to be delayed by at 
least one level, then the type of the formal parameter has to be 
prefixed by the keyword 'lazy'.

If in a called function 'foo' a formal lazy parameter is passed to 
another called function 'bar' as a reference, i.e. without appended 
parentheses, on a position where a lazy parameter is expected, then 
the actual parameter is not evaluated in 'foo'. In all other cases 
of 
reached usage the actual parameter is evaluated.

Example:
int ack( lazy int x, lazy int y){
    // ...
        return ack( x-1, ack( x, y-1));
    // this actual parameter  ^
    // is not evaluated, but passed down only

    // wheras
        return ack( x-1, ack( x(), y-1));
    // this actual parameter    ^
    // _is_ evaluated and then passed down
}


Did I get that correctly?
Oct 12 2006
prev sibling parent reply Ivan Senji <ivan.senji_REMOVE_ _THIS__gmail.com> writes:
Karen Lanrap wrote:

 I have posted the following in the learn-group without any answers:
 
 
 From the documentation:
 
 Lazy arguments are evaluated not when the function is called, but 
 when the parameter is evaluated within the function.
 
 
 First to note, that this is a circular definition without any fixed 
 point to start from:
 a lazy argument is evaluated, when it is evaluated :-(


This is not a circular definition.
import std.stdio;

void func(lazy int x) // x is a parameter
{
   writefln(x()); //parameter evaluated here
   writefln(x()); //and once again here
}


void main()
{
   int x;
   func({return x++;}());

   //{return x++;}() is and argument and it is not called here
}

The thing is: argument and parameter are not one and the same and that 
makes the definition not circular.
Oct 12 2006
parent Karen Lanrap <karen digitaldaemon.com> writes:
Ivan Senji wrote:


 The thing is: argument and parameter are not one and the same
 and that makes the definition not circular.


If it is not circular, then without using a self reference one should 
be able to explain where the point of evaluation is. But i do not see 
such an explanation.

If you are right, that arguemnt and parameter are always not the 
same, then this would mean, that the compiler has to construct a 
delegate even then, when the argument is a delegate already. But then 
a lazy evaluation of the ackermann-function for example would mean, 
that the compiler has to construct a delegate---which's length is 
approximately the length of the number to be output.
Oct 13 2006