Default value of a generic type.

[Herp Derp]

I have a generic class with the @:generic metadata, and under certain circumstances I would like to give a field of the generic type its default value back.

A commented example code:

@:generic

class MyClass<T> {

    public var value: T;

    public function new(value: T) {

        this.value = value;

    }

    public function resetToDefault(): Void {

        // Set the 'value' field to its default value

        // according to its compile-time type.

        // `0` for Int

        // `0.0` for Float

        // `false` for Bool

        // `null` for anything else

        

        // In C# its possible to do it like so:

        this.value = default(T); // invalid Haxe code

        // Done at compilation time with no

        // performance penalty at runtime.

    }

}

Is there a built-in tool that can accomplish this? perhaps its possible to write a macro that will do this?

[Hugh]

On cpp, you can set them to the null-dynamic:

var i:Int = 123;
var d:Dynamic = null; i = d;
trace(i);

You could set it to null directly, but the compiler wont let you, event with "untyped", but this should work:

  untyped i = __cpp__("null()");

Hugh

[Hugh]

Another option might be to create a defaultValue member (or static, if that works with generics) an let it initialize normally, and never set it.  Then just use "value=defaultValue".

Also note that on neko, the integers will start off at null, not 0.

Hugh

[Herp Derp]

On Friday, May 2, 2014 7:22:10 AM UTC+3, Hugh wrote:

On cpp, you can set them to the null-dynamic:

var i:Int = 123;
var d:Dynamic = null; i = d;
trace(i);

You could set it to null directly, but the compiler wont let you, event with "untyped", but this should work:

  untyped i = __cpp__("null()");

I'm not trying to target C++, so this doesn't really help me.

 

On Friday, May 2, 2014 7:26:11 AM UTC+3, Hugh wrote:

Another option might be to create a defaultValue member (or static, if that works with generics) an let it initialize normally, and never set it.  Then just use "value=defaultValue".

Also note that on neko, the integers will start off at null, not 0.

Generic classes can't have static fields, so this won't work. 

[Juraj Kirchheim]

FYI the default value for floats is NaN, not 0.0. In any case, you can
write a macro for that:

class Default {

macro static public function value()
return
switch haxe.macro.Context.follow(haxe.macro.Context.getExpectedType()) {
case TAbstract(t, _):
switch t.toString() {
case 'Int': macro 0;
case 'Bool': macro false;
case 'Float': macro 0.0;
default: macro null;
}
default: macro null;
}
}

//usage:
this.value = Default.value();

Regards,
Juraj

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[Stephane Le Dorze]

Ooooo..

I was not aware of "getExpectedType", that's very good to have it on board! =)

[Herp Derp]

You're right of course about NaN being the default for Float, my mistake.
That is an interesting piece of code, but unfortunately it doesn't work in a generic context.

Apparently it always returns null for a generic type and results in the following error (T is Bool in this case):

On static platforms, null can't be used as basic type Bool

:(

[Herp Derp]

bump

[Juraj Kirchheim]

Can you post a minimal example that reproduces the problem?

On Fri, May 16, 2014 at 7:05 AM, Herp Derp <stupidi...@gmail.com> wrote:
> bump

[Daniel Uranga]

Im interested on this too.

https://gist.github.com/anonymous/915f1948d402676c6b3c is that correct? I get the "On static platforms, null can't be used as basic type Float" error too.

Line 6 "trace(expected)" is printing "TInst(Test.T,[])"; so the switch is defaulting to null.

[Juraj Kirchheim]

Oh, well that's unpleasant. I had hoped macros are executed after
@:generic, which arguably makes more sense. Try filing an issue ;)

[Daniel Uranga]

Well, filled an issue https://github.com/HaxeFoundation/haxe/issues/3022

I had this problem several times trying to write code with @:generic, so it would be really nice to have this feature added.