Erlang : Breaking out of lists:foreach "loop"

Question

I have a list of elements in Erlang, and I'm using lists:foreach to traverse through the elements in the list. Is there a way to break out of this "foreach loop" in the middle of the traversal. For eg: suppose I want to stop traversing the list further if I encounter a '1' in the list [2, 4, 5, 1, 2, 5]. How do I do this?

Answer 1

Another way to do it is to use throw and catch:

catch lists:foreach(
        fun(1) ->
                throw(found_one);
           (X) ->
                io:format("~p~n", [X])
        end,
        [2, 4, 5, 1, 2, 5]).

When run in the shell, this outputs:

2
4
5
found_one

EDIT: By popular demand, a more precise version that catches only what you want to catch:

try lists:foreach(
        fun(1) ->
                throw(found_one);
           (X) ->
                io:format("~p~n", [X])
        end,
        [2, 4, 5, 1, 2, 5])
catch
    throw:found_one ->
        found_one
end.

Answer 2

traverse(Liste) ->
 traverse(Liste, []).

traverse([], Acc) ->
 Acc;    

traverse([1|_], Acc) ->
 Acc;

traverse([H|T], Acc) ->
 % do something useful here maybe?
 traverse(T, Acc).

Of course this is very rough example.

Answer 3

There are many nice functions in lists module:

lists:foreach(fun(E) -> do_something(E) end,
    lists:takewhile(fun(E) -> E =/= 1 end, List)).

or more effective but less nice

lists:takewhile(fun(1) -> false;
                   (E) -> do_something(E), true
                end, List)

Answer 4

I meet the same problem and solve it by this way:

-module(foreach_until).
-export([foreach_until/3]).

foreach_until(Action, L, Judge)     ->
    lists:reverse(foreach_until(Action, L, Judge, []))
    .

foreach_until(_, [], _, Result) ->
    Result
    ;

foreach_until(Action, [H | T], Judge, Result)   ->
    case Judge(H) of 
        true    -> Result;
        false   -> foreach_until(Action, T, Judge, [Action(H) | Result])
    end
    .

Below is an example explained how to use:

60> foreach_until:foreach_until(fun(X) -> X*X end, [1,2,3,4], fun(X)-> X >= 3 end).
[1,4]