http://d.hatena.ne.jp/osiire/20090516#1242456737 のErlang版みたいなもの

card.erl

-module(card).

-export([num/1]).

-export_type([card/0]).

-type card() :: {num, 1..10} | jack | queen | king.

-spec num(card()) -> 1..13.
num({num, N}) -> N;
num(jack)     -> 11;
num(queen)    -> 12;
num(king)     -> 13.

card_ex.erl

-module(card_ex).

-export([num/1, next/1]).

-export_type([card/0]).

-type card() :: card:card() | joker.

-spec num(card()) -> 0..13.
num(joker) -> 0;
num(Other) -> card:num(Other).

-spec next(card()) -> card().
next({num, N}) when N < 10 -> {num, N+1};
next({num, _})             -> jack;
next(jack)                 -> queen;
next(queen)                -> king;
next(king)                 -> joker;
next(joker)                -> {num, 1}.