confusing (buggy?) closure behaviour

[Denis Koroskin]

On Fri, 12 Dec 2008 22:28:01 +0300, Zoran Isailovski  
<dmd.zoc at spamgourmet.com> wrote:

> Denis Koroskin Wrote:
>
>> On Fri, 12 Dec 2008 19:32:03 +0300, Zoran Isailovski
>> <dmd.zoc at spamgourmet.com> wrote:
>>
>> > I'm an experienced C#, Java and Python programmer, and have employed
>> > closures (and C# delegates) upon numerous occasions. While  
>> experimenting
>> > with D closures and delegates, I was stroke by a phenomenon I cannot
>> > explain. Here's the code:
>> >
>> > module closures01;
>> >
>> > import std.stdio;
>> >
>> > alias int delegate(int arg) Handler;
>> >
>> > Handler incBy(int n)
>> > {
>> > 	return delegate(int arg){ return arg + n; };
>> > }
>> >
>> > Handler mulBy(int n)
>> > {
>> > 	return delegate(int arg){ return arg * n; };
>> > }
>> >
>> > void test1()
>> > {
>> > 	writefln("\ntest1:\n----------------------------------------");
>> > 	int x = 10, y;
>> > 	y = mulBy(3)(x); writefln("%d * 3 -> %d", x, y);
>> > 	y = mulBy(4)(x); writefln("%d * 4 -> %d", x, y);
>> > 	y = incBy(2)(x); writefln("%d + 2 -> %d", x, y);
>> > }
>> >
>> > void test2()
>> > {
>> > 	writefln("\ntest2:\n----------------------------------------");
>> > 	int x = 10, y;
>> > 	Handler times3 = mulBy(3);
>> > 	Handler times4 = mulBy(4);
>> > 	Handler plus2 = incBy(2);
>> > 	y = times3(x); writefln("%d * 3 -> %d", x, y);
>> > 	y = times4(x); writefln("%d * 4 -> %d", x, y);
>> > 	y = plus2(x); writefln("%d + 2 -> %d", x, y);
>> > }
>> >
>> > public void run()
>> > {
>> > 	test1();
>> > 	test2();
>> > }
>> >
>> > /* **************************************** *
>> >  * Compiled with: Digital Mars D Compiler v1.030
>> >  *
>> >  * (Unexplainable) program output:
>> > test1:
>> > ----------------------------------------
>> > 10 * 3 -> 30
>> > 10 * 4 -> 40
>> > 10 + 2 -> 12
>> >
>> > test2:
>> > ----------------------------------------
>> > 10 * 3 -> 20
>> > 10 * 4 -> 42846880
>> > 10 + 2 -> 4284698
>> >
>> > * **************************************** */
>> >
>> > What goes wrong???
>>
>> I'd say that it works as expected and here is why.
>>
>> First of all, there are two types of closures:  static and dynamic
>> closures.
>> Closures work by having a hidden pointer to function frame where all  
>> local
>> variables are stored.
>>
>> When a static closure is created, all the function local variables are
>> stored on stack.
>> It has an advantage that no memory allocation takes place (fast).
>> It has a disadvantage that once the delegate leaves the scope, it  
>> becomes
>> invalid since variables were stored on stack and the stack is probably
>> overwritten (unsafe).
>>
>> Dynamic closure allocates memory in a heap and all the local variables  
>> are
>> placed there.
>> It has a disadvantage that memory is allocated for dynamic closure  
>> (might
>> be slow if dynamic closure are created often).
>> It has an advantage that dynamic closure may leave the scope, i.e. you  
>> may
>> save it and call whenever you want.
>>
>> D1 support static closures only! That's why your code doesn't work (in
>> test1 stack is still valid, but in test2 stack gets overwritten)
>> D2 has support for dynamic closures. Just try it - your sample works as  
>> is.
>
> An addition:
>
> Given the complexness of the criteria when a closure works and when not,  
> I would vote for a compiler error on inappropriate closures usage. (In  
> Java, closures cannot handle mutable values on the stack, so it's an  
> error for a method to return a closure that refers to a non-final  
> argument.)

This would restrict their usage so badly that may make then next to  
useless.

No worries, a dynamic closure is created automatically whenever a static  
one it might be unsafe in D2.

As a rule of thumb, you shoudn't don't return local delegate from a  
function in D1, but you may safely pass them down the call stack.