• H. S. Teoh (24/24) Oct 10 2013 I have a template used for storing compile-time values:
• simendsjo (5/32) Oct 10 2013 I've used this horrible hack to solve this. Not very generic as
• Jacob Carlborg (11/32) Oct 11 2013 I found this old thread
• Dicebot (11/11) Oct 11 2013 This is actually a very interesting question.

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

I have a template used for storing compile-time values:

	template Def(int x, string y) {
		alias impl = TypeTuple!(x,y);
	}

How do I define a template isDef that, given some template alias A,
evaluates to true if A is some instantiation of Def?

	template isDef(alias A) {
		enum A = ... /* what to put here? */
	}

The intent is to be able to write signature constraints like this:

	auto myFunc(alias def)(...)
		if (isDef!def)
	{
		...
	}

	...
	// Pass an instantiation of Def to myFunc
	auto x = myFunc!(Def!(1, "abc"))(args);

I tried using std.traits.isInstanceOf but apparently it expects the
second argument to be an actual type, which doesn't work in this case
because Def is a typetuple of compile-time values, not an actual type.


T

-- 
Help a man when he is in trouble and he will remember you when he is in trouble
again.

"simendsjo" <simendsjo gmail.com> writes:

On Thursday, 10 October 2013 at 17:24:37 UTC, H. S. Teoh wrote:
 I have a template used for storing compile-time values:

 	template Def(int x, string y) {
 		alias impl = TypeTuple!(x,y);
 	}

 How do I define a template isDef that, given some template 
 alias A,
 evaluates to true if A is some instantiation of Def?

 	template isDef(alias A) {
 		enum A = ... /* what to put here? */
 	}

 The intent is to be able to write signature constraints like 
 this:

 	auto myFunc(alias def)(...)
 		if (isDef!def)
 	{
 		...
 	}

 	...
 	// Pass an instantiation of Def to myFunc
 	auto x = myFunc!(Def!(1, "abc"))(args);

 I tried using std.traits.isInstanceOf but apparently it expects 
 the
 second argument to be an actual type, which doesn't work in 
 this case
 because Def is a typetuple of compile-time values, not an 
 actual type.


 T

I've used this horrible hack to solve this. Not very generic as 
you can see:
     else static if(T.stringof.length > 13 && T.stringof[0..13] == 
"TupleWrapper!")

Jacob Carlborg <doob me.com> writes:

On 2013-10-10 19:23, H. S. Teoh wrote:
 I have a template used for storing compile-time values:

 	template Def(int x, string y) {
 		alias impl = TypeTuple!(x,y);
 	}

 How do I define a template isDef that, given some template alias A,
 evaluates to true if A is some instantiation of Def?

 	template isDef(alias A) {
 		enum A = ... /* what to put here? */
 	}

 The intent is to be able to write signature constraints like this:

 	auto myFunc(alias def)(...)
 		if (isDef!def)
 	{
 		...
 	}

 	...
 	// Pass an instantiation of Def to myFunc
 	auto x = myFunc!(Def!(1, "abc"))(args);

 I tried using std.traits.isInstanceOf but apparently it expects the
 second argument to be an actual type, which doesn't work in this case
 because Def is a typetuple of compile-time values, not an actual type.

I found this old thread

http://forum.dlang.org/thread/mailman.1382.1371938670.13711.digitalmars-d puremagic.com

With this code:

template getTemplate(alias T : TI!TP, alias TI, TP...)
{
     alias getTemplate = TI;
}

But I haven't figured out how to do the comparison.

-- 
/Jacob Carlborg

"Dicebot" <public dicebot.lv> writes:

This is actually a very interesting question.

Usual approach is to use pattern matching on types but template 
instance symbol is not a type on its own, it is just a symbol. 
(with typeof == void)

My second guess was to check __traits(identifier, AliasParam) but 
it looks like for aliased instance (contrary to aliased template 
symbol) identifier is its mangled name.

I can't imagine really clean way to do it but comparing 
__traits(identifier) to that specific mangled name can be a 
reliable enough hack (better than stringof which has undefined 
format).