A while back, somebody raised the topic of implementing a lazy
range for traversing/flattening a tree structure on #d.
The obvious (in terms of Phobos primitives) solution would be
something along the lines of this:
---
struct Node(T) {
T val;
Node!T[] children;
}
auto flatten(T)(Node!T root) {
import std.algorithm, std.range;
return only(root.val).chain(map!flatten(root.children).joiner);
}
void main() {
alias N = Node!int;
auto a = N(1, [
N(2, [
N(3, [
N(4)
])
]),
N(5)
]);
import std.stdio;
writeln(a.flatten); // [1, 2, 3, 4, 5]
}
---
But of course, that piece of code does not compile with current
DMD, as the return type of flatten() can't refer to the function
itself (during name mangling).
Now, one way around this would be to add an array() call at the
end of the return statement, which hides the type of map!flatten,
but at the cost of many unnecessary memory allocations. A second
option would be to use inputRangeObject to convert the return
value to ForwardRange!T (well, that is if it actually worked, due
to an implementation bug it leads to a runtime crash).
But can you think of a more simple/elegant workaround?
(Note aside: Obviously, the fact that the code relies on
recursion might be an issue, and a simple opApply-based solution
with a worklist stack would likely perform better. Still, I think
it's a simple, yet interesting problem.)
David
