import std.stdio;
import std.typetuple;
import std.conv;
import std.traits;

template Cat(T...) {
    template Impl(string cont, uint n, T...) {
        static if(T.length == 1) {
            const string Impl = cont ~ "T[" ~ n.to!string ~ "] f" ~ n.to!string ~ ";}";
        } else {
            const string Impl = Impl!(cont ~ "T[" ~ n.to!string ~ "] f" ~ n.to!string ~ ";", n + 1, T[1 .. $]);
        }
    }
    mixin(Impl!("static struct Container {", 0, T));
    alias Cat = Container;
}

template CatRawGet(Cat, uint n) {
    alias CatRawGet = FieldTypeTuple!(Cat)[n];
}

template CatGet(C, F) {
    alias T = FieldTypeTuple!C;
    alias Field = T[0];
    alias Value = T[1];
    static if(T.length == 2) {
        static if(is(F == Field)) {
            alias CatGet = Value;
        } else {
            static assert(0, "Field " ~ F.stringof ~ " not found.");
        }
    } else static if(is(F == Field)) {
        alias CatGet = Value;
    } else {
        alias CatGet = CatGet!(Cat!(T[2 .. $]), F);
    }
}

template CatSet(C, F, V) {
    alias T = FieldTypeTuple!C;
    pragma(msg, FieldTypeTuple!C);
    alias Field = T[0];
    alias Value = T[1];
    static if(T.length == 2) {
        static if(is(F == Field)) {
            alias CatSet = Cat!(F, V);
        } else {
            static assert(0, "Field " ~ F.stringof ~ " not found.");
        }
    } else static if(is(F == Field)) {
        alias CatSet = Cat!(F, V, Cat!(T[2 .. $]));
    } else {
        alias CatSet = Cat!(Field, Value, FieldTypeTuple!(CatSet!(Cat!(T[2 .. $]), F, V)));
    }
}

struct PairLeft { }
struct PairRight { }

template PairNew(X, Y) {
    alias PairNew = Cat!(PairLeft, X, PairRight, Y);
}

struct N0 { }
struct N1 { }

template Succ(Num) {
    static if(is(Num == N0)) {
        alias Succ = N1;
    } else static if(is(Num == N1)) {
        alias Succ = PairNew!(N1, N0);
    } else static if(is(CatGet!(Num, PairRight) == N0)) {
        alias Succ = PairNew!(CatGet!(Num, PairLeft), N1);
    } else static if(is(CatGet!(Num, PairRight) == N1)) {
        alias Succ = PairNew!(Succ!(CatGet!(Num, PairLeft)), N0);
    } else {
        static assert(0, "Err");
    }
}

void main() {
    alias Q = PairNew!(PairNew!(PairNew!(N1, N1), N1), N1);
    alias R = Succ!(Q);
    alias P = PairNew!(PairNew!(PairNew!(PairNew!(N1, N0), N0), N0), N0);
    static assert(is(R == P));
}