2013/8/13 monarch_dodra <monarchdo...@gmail.com>
> Related: I have encountered this problem, and I can't seem to work around
> it; *other* than non-parameterized templates. Basically, I have this pred
> function, we'll call it "foo". This pred function can itself be
> parameterized to take its own (optional) pred. This basically allows:
> foo(a, b)
> or
> foo!pred(a, b)
>
> This is "traditionally" solved by doing:
> void foo(A, B)(A a, B b);
> void foo(alias pred, A, B)(A a, B b);
>
> Here is the kicker though: "foo" is itself a pred. This means that I
> *need* to be able to pass "foo!pred" as a predicate. This does not work, as
> "foo!pred" is nothing: The compiler doesn't know what you are talking
> about, as the parameters A and B are missing. This is usually solved by a
> template:
>
> template foo(alias pred)
> {
> void foo(A, B)(A a, B b);
> }
>
> This works.... *however* the presence of the "void foo(A, B)(A a, B b);"
> confuses the crap out of the compiler:
> foo(a, b); //OK
> alias PRED = foo!"hello";
> PRED(a, b); //OK
>
> BUT:
> foo!"hello"(a, b); //Error:
> Error: template hello.foo does not match any function template
> declaration. Candidates are:
> hello.foo(R1, R2)(R1 r1, R2 r2)
> hello.foo(alias pred)
> Error: template hello.foo(R1, R2)(R1 r1, R2 r2) cannot deduce template
> function from argument types !("hello")(int, int)
> Error: template instance foo!"hello" errors instantiating template
>
> So... how to make this work? AFAIK, non-parameterized templates should
> solve it. Or is it just a compiler issue?
>
> FYI: This is a problem present in Phobos. You can use:
> "equal!equal(RoR1, RoR2)"
> To compare two ranges of ranges. equal!equal gets instanciated, because
> the args are present to "finish" the missing R1/R2 args after pred.
>
> However, this neat trick stop there:
> "equal!(equal!equal)(RoRoR1, RoRoR2)"
> This time, this does not work, as the compiler can't resolve the predicate.
>
> I'd like to solve this. IMO "equal!equal" is one of the neatest "1-word"
> in D, and I want to make sure it works as one would expect it to.
>
> --------
>
> So: Any workaround recommendations? Alternatives? Thoughts on
> parameter-less templates (regardless of my issue)?
>
Maybe: http://d.puremagic.com/issues/show_bug.cgi?id=10811
Kenji Hara
