Simple trampoline code

[Ellery Newcomer]

bearophile wrote:
> I'm trying to convert to D2 the following (quite simplified up) Python code, that implements a trampoline to run tail-call functions with no stack overflow:
> 
> # Python code
> # *args means almost all the arguments
> def trampoline(fun, *args):
>     thunk = lambda : fun(*args)
>     while True:
>         (thunk, result) = thunk()
>         if thunk is None:
>             return result
> 
> # a tail-recursive function
> def summer(n, p = 1):
>     if n >= 2:
>         return (lambda n=n, p=p: summer(n-1, n+p), None)
>     else:
>         return (None, p)
> 
> assert trampoline(summer, 1000) == 500500
> 
> 
> My D2 version so far (doesn't work):
> 
> // D2 + Phobos2 code
> import std.typecons: tuple;
> import std.stdio: writeln;
> 
> int trampoline(TyFun, TyTuple)(TyFun fun, TyTuple args) {
>     auto thunk = { fun(args.tupleof) };
>     while (true) {
>         auto pair = thunk();
>         thunk = pair.field[0];
>         auto result = pair.field[1];
>         if (thunk is null)
>             return result;
>     }
> }
> 
> auto summer(int n, int p=1) {
>     if (n >= 2)
>         return tuple((int n=n, int p=p){return summer(n-1, n+p);}, 0);
>     else
>         return tuple(null, p);
> }
> 
> void main() {
>     writeln(trampoline(summer, tuple(1000)));
> }
> 
> The D2 compiler outputs:
> trampoline.d(18): Error: forward reference to inferred return type of function call summer(n - 1,n + p)
> Can that D2 code be fixed?
> 
> Bye,
> bearophile

How DO you define the signature of a function that returns itself?

And FYI, dmd handles your particular example recursively just fine. But
you probably know that.

That aside, this is about the best that I can get:

int trampoline (TyFun, TyTuple) (TyFun fun, TyTuple targs){
    while (1){
        auto pair = fun(targs.tupleof[0..2]);
        fun = pair.field[0];
        int result = pair.field[1];
        targs = tuple(pair.tupleof[2..4]);
        if(fun is null){
            return result;
        }
    }
}

class Durr{
    alias summer opCall;
    Tuple!(Durr,int,int,int) summer(int n, int p){
        if (n>= 2){
            return tuple(this,0,n-1,n+p);
        }else return tuple(cast(Durr)null,p,0,0);
    }
}