• Jeroen Dirks <jeroen.dirks sympatico.ca> Jun 17 2009
• BCS <ao pathlink.com> Jun 17 2009
• Walter Bright <newshound1 digitalmars.com> Jun 17 2009
• div0 <div0 users.sourceforge.net> Jun 18 2009
• bearophile <bearophileHUGS lycos.com> Jun 17 2009
• Jason House <jason.james.house gmail.com> Jun 17 2009
• bearophile <bearophileHUGS lycos.com> Jun 17 2009
• Jason House <jason.james.house gmail.com> Jun 18 2009
• Don <nospam nospam.com> Jun 30 2009
• "Denis Koroskin" <2korden gmail.com> Jun 18 2009
• bearophile <bearophileHUGS lycos.com> Jun 18 2009
• Jeroen Dirks <jeroen.dirks sympatico.ca> Jun 18 2009
• Jesse Phillips <jessekphillips gmail.com> Jun 18 2009

Jeroen Dirks <jeroen.dirks sympatico.ca> writes:

Does anyone know the answer to this D related question on SO?

http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/

BCS <ao pathlink.com> writes:

Reply to Jeroen,

 Does anyone know the answer to this D related question on SO?
 
 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/
 


DMD can't tell that the method call is safe even though it is inpure.

Walter Bright <newshound1 digitalmars.com> writes:

BCS wrote:
 Reply to Jeroen,
 
 Does anyone know the answer to this D related question on SO?

 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/


 DMD can't tell that the method call is safe even though it is inpure.


Also, the purity of a function is determined by its signature, not by 
its implementation.

div0 <div0 users.sourceforge.net> writes:

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Walter Bright wrote:
 BCS wrote:
 Reply to Jeroen,

 Does anyone know the answer to this D related question on SO?

 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/


 DMD can't tell that the method call is safe even though it is inpure.


 Also, the purity of a function is determined by its signature, not by
 its implementation.


Surely the heap itself is global state?
So creating a new object should be forbidden.

- --
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
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bearophile <bearophileHUGS lycos.com> writes:

Jeroen Dirks:
 Does anyone know the answer to this D related question on SO?
 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/


I like this answer:
you may have found case where the compiler's semantic analysis is not as good
as your brain's.<


"pure" is something that has being added recently to D, so its semantics isn't
fully refined yet. Walter & Don will hopefully keep working on it (adding
optimizations, like pulling out pure calls from loops, adopting some strategy
to keep purity even regarding changes in FP rounding modes, improving purity
analysis for classes, etc) removing some of the current faults/limits.

But we have to keep looking for corner cases and possible bugs.
This is something I have written recently, Don may be interested:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D.learn&article_id=16907

Bye,
bearophile

Jason House <jason.james.house gmail.com> writes:

Jeroen Dirks Wrote:

 Does anyone know the answer to this D related question on SO?
 
 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/


addMsg is not marked pure and pure functions can't call impure functions... I'm
not sure if addMsg can be marked pure or not. It does not touch global state
but does have side effects. The key is if side effects to on the containing
object or not. (I haven't used pure enough to know. Many functions in Phobos
are not marked as pure even though they could be. That limits pure's use for me)

bearophile <bearophileHUGS lycos.com> writes:

Jason House:
Many functions in Phobos are not marked as pure even though they could be. That
limits pure's use for me)<


If you list them then Walter may later fix Phobos.

Bye,
bearophile

Jason House <jason.james.house gmail.com> writes:

bearophile Wrote:

 Jason House:
Many functions in Phobos are not marked as pure even though they could be. That
limits pure's use for me)<


 If you list them then Walter may later fix Phobos.


I did file an entry I'm bugzilla for std.cpuid. It was marked as a dup of the
entry for std.math and then only std.math got fixed. I should probably reopen
the bug report

Don <nospam nospam.com> writes:

Jason House wrote:
 bearophile Wrote:
 
 Jason House:
 Many functions in Phobos are not marked as pure even though they could be.
That limits pure's use for me)<




 I did file an entry I'm bugzilla for std.cpuid. It was marked as a dup of the
entry for std.math and then only std.math got fixed. I should probably reopen
the bug report
 


std.cpuid will be removed. The functionality is going into druntime.

"Denis Koroskin" <2korden gmail.com> writes:

On Thu, 18 Jun 2009 00:32:20 +0400, Jeroen Dirks  
<jeroen.dirks sympatico.ca> wrote:

 Does anyone know the answer to this D related question on SO?

 http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/


There needs to be a way to tell that addMsg doesn't access global  
variables.

One of the solutions is to add one more annotation (I call it impure here,  
because it is an exact opposite of pure):

void foo() // doesn't access globals, fine to call inside pure functions
{
}

void bar() // fails to compile, accesses global state but doesn't have  
proper annotation
{
     errno = 42;
}

impure void bar() // fine
{
     errno = 42;
}

But there might be a problem: calling impure function makes your function  
impure, too, so it's viral.

Another solution is... remove global variables entirely! No globals - no  
problems :)

bearophile <bearophileHUGS lycos.com> writes:

Denis Koroskin:
There needs to be a way to tell that addMsg doesn't access global variables.<
Another solution is... remove global variables entirely! No globals - no 
problems :)<


"global variables" are names in the outer scope for nested functions, so that
doesn't solve the problem, and generally kills C compatibility of D.

Time ago I have suggested a keyword like uses or outer, to explicitly list what
names a function uses from the outer scope:

int z;
int bar(int k) uses out z {
    int y = k * 2;
    int foo(int x) uses in y, out z {
        z = y * 2;
        return x * y;
    }
    return k + foo(k * 3);
}

But this example is a mess already, better to not encourage the usage of such
coding style.

Bye,
bearophile

Jeroen Dirks <jeroen.dirks sympatico.ca> writes:

It seems that we are back to making copied all over the place because
the compiler can not tell if we would modify something shared.

If the compiler would be smarter and maybe with the help of some
annotations on the functions called can figure out what data is owned
by a pure operation then you might have a real killer feature here.

1 Compiler guarantees related to locality of changes (without need for
locks everywhere)
2 Still write imperative style code.
3 Not having to do copy on write all the time so that old object is
not changed.

Maybe something like this could be added:

class TestObj
{
   init( int v )
   {
      value = v;
   }
   int value;
}

class Log
{
   internal string[] msg;
   internal TestObj[] values;
   local addMsg( string s ) // marked local since it only changes
internal data
   {
      msg ~= s;
   }
   local addValue( int v )
   {
      values != new TestObj( v )
   }
   local incValues()
   {
      for ( v; values ) {
         v.value += 1;
      }
   }
   pure string[] getMsgs() {
      return msg.dup();  // have to make a copy since it is internal
   }
   // following would be compiler errors:
   string[] getMsg2()
   {
      return msg;  // error: references to internal can not be
returned or exported somehow
   }
   local addValue2( TestObj t )
   {
      values ~= tl // error: can not assign external object to
internal data
   }
};

pure Log WorkWithObj()
{
   Log l = new Log();
   l.addMsg("Hello");
   l.addMsg("World");
   l.addValue( 41 );
   l.addValue( 9 );
   l.incValues();
   return l;
}

Jesse Phillips <jessekphillips gmail.com> writes:

On Thu, 18 Jun 2009 23:21:29 +0100, div0 wrote:

 
 Surely the heap itself is global state? So creating a new object should
 be forbidden.
 


Isn't the stack just as global as the heap? By global it isn't referring 
to storage location but the scoping that is enforced by the compiler.