Question

I need to break from foldl. Here is a dummy example how to break from fold when I count sum of values in a list and meet too big value (i.e. 10)

   L = [1,2,3,4,10,5,6,7],

   Res = 
      try
         lists:foldl(
            fun(I, Value) ->
               if (I < 10) ->
                  Value + I;
               true ->
                  throw({too_big_value, Value})
               end
            end,
            0, L)
      catch
         throw:{too_big_value, Value} -> Value
      end,

   Res.

I know this example is artificial but are there any nice method to break out fold (I know that fold always scan the whole structure)?

Please note, that i need to retrieve correct data even if i break from fold. In this case i should get data from previous iteration (as it done in my example).

Answer 1

You’re doing it right, using a throw with try/catch for nonlocal return. If the function looked at the return value from the fun to decide whether or not to continue, it wouldn’t be foldl anymore.