I wrote:
you would want to use foldr, not foldl
Imants Cekusins wrote:
foldl enumerates items in "natural" order: in [1,2] 1 is passed to the fn, then 2 foldr on the other hand begins with 2 and ends with 1.
The list is not reversed by either one of them. And in both of them, the first element of the list is examined first. With foldl, you apply the complete operation to each element of the list in turn, all the way to the end of the list. With foldr, you start with the first element and say: Let's think about what will happen when we apply the operation to the first element with the result of the entire fold from the second element onward. Knowing only the first element at this time, can we already say what the result will be? In this case, there are two possibilities: If the first element is Just, then yes, we already know the result, and we can discard the rest of the fold without ever actually computing it. If the first element is Nothing, then we don't know the result yet. But we do know that the first element is irrelevant, so we discard it and move on. Doesn't foldr seem like a more natural way to do what you want here? The reason this doesn't work in Erlang is because Erlang is a strict language. When we try to apply the operation with first parameter the first element of the list and second parameter the rest of the fold, Erlang does the entire calculation of that second parameter - the entire fold - before it even starts thinking about the operation. Regards, Yitz