Convert clause to a Fun

Question

How to represent this clause in one line using Fun.

perms([]) -> [[]];
perms(L) -> [[H|T] || H <- L, T <- perms(L--[H])].

Answer 1

I believe what you are seeking is for a fun to be "self-recursive".

The fun syntax is not able to refer to itself inside the fun body, so one need to use a trick where the fun to call is a parameter. This is commonly referred to as the ycombinator.

Some example code will likely describe it better:

permutator() ->
  fun 
    ([], _F) ->
      [[]];
    (L, F) ->
      [ [H|T] || H <- L, T <- F(L--[H], F)]
  end.

do_permutate(L) ->
  P = permutator(),
  P(L, P).

As you can see this is quite awkward. If you just wanted to refer to the perms functions of yours, you can use the code: fun perms/1.

Answer 2

I also got another answer similar to Christian.

5> Perms = fun(X) -> Fun = fun([],F) -> [[]]; (L,F) -> [[H|T] || H <- L, T <- F(L--[H],F)] end, Fun(X, Fun) end.
#Fun<erl_eval.6.13229925>
6> Perms("cat").
["cat","cta","act","atc","tca","tac"]