Problem Statement

The set of combinations with repetitions is computed from a set,

S

{\displaystyle S}

(of cardinality

n

{\displaystyle n}

), and a size of resulting selection,

k

{\displaystyle k}

, by reporting the sets of cardinality

k

{\displaystyle k}

where each member of those sets is chosen from

S

{\displaystyle S}

. In the real world, it is about choosing sets where there is a “large” supply of each type of element and where the order of choice does not matter. For example: Note that both the order of items within a pair, and the order of the pairs given in the answer is not significant; the pairs represent multisets. Also note that doughnut can also be spelled donut.

Let's start with the solution:

-module(comb).
-compile(export_all).

comb_rep(0,_) ->
    [[]];
comb_rep(_,[]) ->
    [];
comb_rep(N,[H|T]=S) ->
    [[H|L] || L <- comb_rep(N-1,S)]++comb_rep(N,T).