IMO it’s even simpler to use a monad-type extension to functions to suspend state. (This can be coroutines/generators, explicit Haskell monads, some macros enca

IMO it’s even simpler to use a monad-type extension to functions to suspend state. (This can be coroutines/generators, explicit Haskell monads, some macros encapsulating longjmp or whatever.) queue <- { root } while queue is not empty node <- dequeue one node # One of the following: # for a simple function approach… if predicate(node) return node # or for a monadic approach, can be overloaded generically… yield! node for child of node enqueue child The monad-style extensions fits with the other pieces of the language much more naturally IMO. You could easily use a predicate monad to regain the functionality of the naive implementation, or a more complex monad if you want a complex BFS traversal, but the function looks almost identical either way. Python and Ruby and Lua and JavaScript even do this! C++20 will probably make this somewhat more common too. (The word “monad” may set off sirens here, but it really just refers to a feature similar to but somewhat more generic than “yield” in a coroutine/generator.)