.
+-spec put(A, fun((A) -> boolean())) ->
+ t(A).
put(X, F) ->
case F(X)
of true -> {some, X}
; false -> none
end.
+-spec get(t(A), Default :: A) ->
+ A.
get({some, X}, _) -> X;
get(none , Y) -> Y.
+-spec map(t(A), fun((A) -> (B))) ->
+ t(B).
map({some, X}, F) -> {some, F(X)};
map(none , _) -> none.