Is there any way to fix this elegant way to walk a tree at compile time?

Gareth Charnock gareth.charnock at
Mon Jan 17 17:30:19 PST 2011

I was trying to walk a tree at compile time and I came up with this 
method. It seems very elegant, but unfortunately it fails with a 
recursive alias declaration error. I have a bad feeling there's no way 
to fix this but I thought I'd ask in case anyone had any ideas.

( for syntax highlighting, easier on the eye)

import std.typetuple;

template staticReduce(alias F,T...) {
	static if(T.length==0) {
		alias TypeTuple!() staticReduce;
	} else static if(T.length==1) {
		enum staticReduce=T[0];
	} else {
	    enum staticReduce=staticReduce!(F,TypeTuple!(F!(T[0],T[1]),T[2..$]));

template concan(string s1,string s2) {enum concan = s1 ~ s2;};
static assert(staticReduce!(concan,"Hello", " world")=="Hello world"); 

template Flatten(Tree,string s = "") {
	alias staticReduce!(concan,staticMap!(Flatten,Tree.ChildrenTuple)) Flatten;

struct TreeNode(Children...) {
	alias Children ChildrenTuple;

alias TreeNode!("A",TreeNode!("B","C","D"),"E") StaticTree;
static assert(Flatten!(StaticTree) == "ABCDE");
//Error: alias 
recursive alias declaration

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