template deduction

[bearophile]

Mafi:
> I am not able to instatiate ths template in any way.

You are doing it wrong. Such template tricks are not easy to get right, so probably the best thing you may do is to start from an empty program that works, and then keep adding to it, and keep it working, until you reach something good enough. If you write ten lines of code then later it's hard to make them work :-)

This is an implementation that seems to work, but it's not nice, maybe you can improve it, I have had to use a delegate because I have found a forward reference problem (I am not sure if it's worth for Bugzilla):


import std.stdio: writeln;
import std.typecons: Tuple, tuple;
import std.traits: ReturnType, ParameterTypeTuple;

struct Memoize(alias F) {
    ReturnType!F opCall(ParameterTypeTuple!F args) {
        alias ReturnType!F ResultType;
        static ResultType[Tuple!(ParameterTypeTuple!F)] cache;
        auto ptr = tuple(args) in cache;
        if (ptr == null) {
            auto result = F(args);
            cache[tuple(args)] = result;
            return result;
        } else {
            return *ptr;
        }
    }
}

pure int fib(int n) {
    if (n < 2)
        return n;
    return fib(n - 1) + fib(n - 2);
}

void main() {
    pure int delegate(int n) cfib_impl;
    Memoize!cfib_impl cfib;

    cfib_impl = (int n) {
        if (n < 2)
            return n;
        return cfib(n - 1) + cfib(n - 2);
    };

    enum int N = 40;
    writeln("fib(", N, ") = ", fib(N));
    writeln("cfib(", N, ") = ", cfib(N));
}

Bye,
bearophile