I think he meant that the inner function produces an Option, structurally speaking. A Just or a Nothing.
dogweather
0
dogweather
I forget about flat_map and it’s great for this task. It embodies the “map, but maybe only keep some”, like you say. So as the coder, I’m on alert as to what’s happening. Vis-a-vis reduce which is totally open-ended structure creation. And so its presence doesn’t give any kind of ‘hint’ to the reader.
1
dogweather
Nice, so updating with flat_map, my current extracted function is:
def cat_oks(list, fun) do
list
|> Enum.flat_map(fn
{:ok, result} -> [result]
{:error, msg} -> fun.(msg); []
end)
end
1
cloudytoday
Since this is a “good pattern” thread… shouldn’t the name of the function represent that it’s also doing something with errors rather than just filtering for oks? 
0
al2o3cr
What @dogweather said, though I’d spell it like this:
[x]isSome(x)[]isNone
This matches the behavior of Option in Rust’s implementation.
1
cloudytoday
Interesting, I haven’t yet seen implementations of Option in terms of lists, only in terms of tagged unions. Thanks for pointing out!
0
ityonemo
I guess people hate typing out those pesky tuples but I like having to do it (and don’t super love options types) because it doesn’t ‘hide’ the underlying data structure and thus makes reading the code have less mental overhead.
2