Robert Jacques 
Posted in reply to Gareth Charnock
| On Tue, 02 Nov 2010 22:03:47 -0400, Gareth Charnock <gareth.charnock@gmail.com> wrote:
> I've been trying to correctly implement the interpreter patten/expression templates in D (for reference this is a summary of the C++ interpreter patten can be found here http://www.drdobbs.com/184401627). I've run into a problem and I'm not sure if it's a compiler bug or not. The testcase is:
>
> struct BinaryOp(L,string op,R) {
> pragma(msg,"Instansiating " ~ typeof(this).stringof);
> BinaryOp!(typeof(this),s,R1) opBinary(string s,R1)(R1 r) {
> pragma(msg,"Instansiating BinaryOp.opBinary ~L.stringof ~ op ~ R1.stringof);
> return typeof(return)();
> }
> }
>
> struct Leaf {
> BinaryOp!(typeof(this),s,R) opBinary(string s,R)(R r) {
> pragma(msg,"Instansiating leaf.opBinary(" ~ R.stringof ~ ")");
> return typeof(return)();
> }
> };
>
> void main() {
> Leaf v1,v2,v3;
> pragma(msg,"");
> pragma(msg,"======= This Compiles ======");
> v1*(v2*v3);
> pragma(msg,"");
> pragma(msg,"======= This Doesn't ======");
> (v1*v2)*v3;
> }
> Output:
> ======= This Compiles ======
> Instansiating BinaryOp!(Leaf,s,Leaf)
> Instansiating leaf.opBinary(Leaf)
> Instansiating BinaryOp!(Leaf,s,BinaryOp!(Leaf,s,Leaf))
> Instansiating leaf.opBinary(BinaryOp!(Leaf,s,Leaf))
>
> ======= This Doesn't ======
> Error: recursive template expansion for template argument BinaryOp!(Leaf,s,Leaf)
>
> I've tracked the problem down to the return type of BinaryOp.opBinary. Clearly putting BinaryOp!(typeof(this),...) would be a Bad Thing in the main template body but opBinary is a template that may or may not be instantiated so it shouldn't automatically lead to runaway instantiation. It seems the compiler is a little bit overzealous in making sure that such runaway instantiations do not happen.
>
> Is this a bug? Should I file it? Here's what I think a minimal test case might look like:
>
> struct A(T1) {
> void templateFunc(T2)(T2 a) {
> alias A!(typeof(this)) error;
> }
> }
>
>
> void main() {
> A!int a;
> a.templateFunc!int(0);
> }
>
I'm going to lean on the side of this being a compiler bug (so please file), as there are multiple workarounds without logically changing anythings
Here's one:
struct BinaryOp(alias L,string op, R) {
pragma(msg,"Instansiating ", typeof(this).stringof);
BinaryOp!(BinaryOp,s~"",R1) opBinary(string s, R1)(R1 r) {
pragma(msg,"Instansiating BinaryOp.opBinary", L.stringof, " ", op," ",R1.stringof);
return typeof(return)();
}
}
struct Leaf {
BinaryOp!(Leaf,s~"",R) opBinary(string s,R)(R r) {
pragma(msg,"Instansiating leaf.opBinary(", R.stringof, ")");
return typeof(return)();
}
};
And here's another
struct BinaryOp(L,string op, R) {
pragma(msg,"Instansiating ", typeof(this).stringof);
BinaryOp!(BinaryOp,s,R1) opBinary(string s, R1)(R1 r) {
pragma(msg,"Instansiating BinaryOp.opBinary", L.stringof, " ", op," ",R1.stringof);
return typeof(return)();
}
}
struct Leaf {
BinaryOp!(Leaf,s~"",R) opBinary(string s,R)(R r) {
pragma(msg,"Instansiating leaf.opBinary(", R.stringof, ")");
return typeof(return)();
}
};
In general, when passing template value parameters to another template, I'd recommend performing a no-op on them (i.e. ~"" or +0), since sometimes they're passed as N or op instead of 10 or "+". Also, your can use the template name inside it to refer to that instance's type (i.e. you don't have to use typeof(this)).
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