A implementation for generating permutations using fixed-head approach

permutations_fixed_head.ml

let rm x l = List.filter ((<>) x) l

let rec permutations = function
  | [] -> []
  | x::[] -> [[x]]
  | l -> 
    List.fold_left (fun acc x -> acc @ List.map (fun p -> x::p) (permutations (rm x l))) [] l
    
(* The List.fold_left makes every element have their oppotunities to be a head *)