Snippet from https://play.nim-lang.org

playground.nim

import algorithm, sequtils

proc permutations(v: seq[int]): seq[seq[int]] =
  let nbMaxPerms = toSeq(1..v.len).foldr(a * b) # nb permutations max = (len v)!
  result = @[v]
  var temp = v
  for cpt in 1..<nbMaxPerms:
    temp.nextPermutation()
    result.add(temp)

var v = toSeq(1..5)
echo permutations(v)
echo v