Possible improvement of phobos map?

[JG]

On Monday, 4 July 2022 at 19:44:11 UTC, JG wrote:
> Hi,
>
> There was a recent discussion regarding suggesting some special 
> algorithms for associative arrays, which made we wonder if the 
> problem isn't actually that it is a slightly annoying when 
> working with ranges of tuples or structs.
>
> For instance you might have code like this
>
> ```d
>   auto a = ["a":5, "b":6];
>   a.byPair.map!(t=>format!"%s=%s"(t[0],t[1])).writeln;
> ```
> or
> ```d
>   auto r = iota(1,10);
>   auto gcds = cartesianProduct(r,r).map!(t=>gcd(t[0],t[1]));
>
> ```
> which while not too bad isn't as readable as it could be, and 
> it gets worse when the chain gets longer.
>
> With some slight changes to map (which wouldn't be needed if 
> pattern matching exists), one can do this:
>
> ```d
> import std;
>
> struct MapPrototype(alias f, R) {
>     R r;
>     bool empty() { return r.empty; }
>     auto front() { return f(r.front.tupleof); }
>     void popFront() { r.popFront; }
>     auto save() { return typeof(this)(r.save); }
> }
>
> auto mapPrototype(alias f, R)(R r) {
>     return MapPrototype!(f,R)(r);
> }
>
> struct Point {
>     int x;
>     int y;
> }
>
> void main() {
>     auto r = iota(1,10);
>     auto gcds = cartesianProduct(r,r).mapPrototype!(gcd);
>     gcds.writeln;
>     auto a = ["a":5, "b":6];
>     
> a.byPair.mapPrototype!((name,value)=>format!"%s=%s"(name,value)).writeln;
>     
> [Point(1,2),Point(3,4)].mapPrototype!((x,y)=>x*x+y*y).writeln;
> }
> ```
>
> Thoughts?

I should have added, this is incomplete code one needs to check 
which behavior is wanted decomposition like above or the old 
behavior.