Erlang组合学

Question

I want to find combinatorics of integer in Erlang, equivalent of this Haskell code:

composition 0 = [[]]
composition n = [x:rest | x <- [1..n], rest <- composition (n-x)]

so far I tried this which It doesn't work:

-module(combinatorics).
-export([comb/1]).
comb([]) -> []; 
comb(N) ->
        [[X|R] || X <- lists:seq(1,N),R=comb(N-X)].

The code return This error:

43> combinatorics:comb(2).
     ** exception error: no case clause matching []
             in function  combinatorics:'-comb/1-lc$^0/1-0-'/2 (combinatorics.erl, line 5)
             in call from combinatorics:'-comb/1-lc$^0/1-0-'/2 (combinatorics.erl, line 5)

Answer 1

You have multiple errors in the translation:

1. The first clause should be comb(0) -> [[]];

2. You've done R = comb(...) instead of R <- comb(...) in the list comprehension.

With these changes, the code works:

-module(combinatorics).
-export([comb/1]).
comb(0) -> [[]];
comb(N) ->
    [[X|R] || X <- lists:seq(1,N), R <- comb(N-X)].

1> c(combinatorics).
{ok,combinatorics}
2> combinatorics:comb(1).
[[1]]
3> combinatorics:comb(2).
[[1,1],[2]]
4> combinatorics:comb(3).
[[1,1,1],[1,2],[2,1],[3]]
5> combinatorics:comb(4).
[[1,1,1,1],[1,1,2],[1,2,1],[1,3],[2,1,1],[2,2],[3,1],[4]]