Convert little imperative code to functional coding style

[Timon Gehr]

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import std.stdio;
import std.range        : iota;
import std.algorithm    : map, filter, joiner;
import std.typecons     : tuple;
import std.math         : sqrt, floor;

void main(){
     immutable limit = cast(size_t)floor(sqrt(1_000.0));

     auto r = iota(2,limit).map!(m=>iota(1,m-1).map!(n=>tuple(m,n))).joiner
             .filter!(t=>2*t[0]*(t[0]+t[1])==1_000)
             .map!(t=>(t[0]^^4-t[1]^^4)*(2*t[0]*t[1]));

     writeln(r.front);
}