[Issue 4167] New: Template overrides with alias

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4167

           Summary: Template overrides with alias
           Product: D
           Version: future
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-05-09 07:02:05 PDT ---
This is an enhancement request, but I am not sure if this can be done, and even
if it's doable I don't know if this is a good idea. If it's hard to implement
or it's not a good idea you can close this.

This is correct D2 code that uses an alias to emulate a new override of
function foo:


int foo(int x) {
    return 10;
}
int bar(int x, int y) {
    return 20;
}
alias bar foo;
void main() {
    int x1 = foo(1);      // OK
    int x2 = bar(2, 100); // OK
    int x3 = foo(3, 100); // OK
}



But similar code with templates doesn't work, and produces (dmd 2.043):
test.d(7): Error: alias test.Foo conflicts with template test.Foo(int x) at
test.d(1)


template Foo(int x) {
    enum int Foo = 10;
}
template Bar(int x, int y) {
    enum int Bar = 10;
}
alias Bar Foo; // line 7, Error
void main() {
    int x1 = Foo!(1);      // OK
    int x2 = Bar!(2, 100); // OK
    int x3 = Foo!(3, 100); // Not OK
}



To do that I have to create a wrapper, this seems to work:

template Foo(int x) {
    enum int Foo = 10;
}
template Bar(int x, int y) {
    enum int Bar = 10;
}
template Foo(int x, int y) {
    enum int Foo = Bar!(x, y);
}
void main() {
    int x1 = Foo!(1);      // OK
    int x2 = Bar!(2, 100); // OK
    int x3 = Foo!(3, 100); // OK
}


Is it possible to change the compiler so an alias can be used to emulate
template overloads?

I have asked on IRC and two persons have said that such usage of alias with
templates can be a little confusing.

This usage of the alias can be seen useful for a simple form of Partial
Template Application, similar to template currying:


import std.stdio: writeln;
import std.conv: to;
import std.algorithm: array, map;

template to_(Tout) {
    Tout to_(Tin)(Tin x) {
        return to!(Tout, Tin)(x);
    }
}
alias to to_; // Currently an error here
void main() {
    string[] data = ["1", "2", "3"];
    int[] arr = array(map!(to_!int)(data));
    writeln(to_!int("10"));
}


Currently the map! needs a full template specialization:
map!(to!(int, string))(data)

While the writeln can use half specialization:
writeln(to!int("10"));

Defining that to_ plus using an alias I can create a partial template.

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