Imperative and purely functional depth-first searches in F#

dfs.fs

open System.Collections.Generic

/// Imperative depth-first search
let dfs (V, E) =
  let visited = HashSet(HashIdentity.Structural)
  let stack = Stack[V]
  while stack.Count > 0 do
    let u = stack.Pop()
    if not(visited.Contains u) then
      for v in E u do
        stack.Push v
      visited.Add u
      |> ignore

/// Functional depth-first search as a fold
let dfs f a (V, E) =
  let rec loop visited a = function
    | [] -> a
    | u::us ->
        if List.contains u visited then loop visited a us else
          loop (u::us) (f a u) (E u @ us)
  loop a V