I'm doing doing "map" over a collection and getting back a list of maps:
res1 = Enum.map(my_collection, fn a -> %{a.id => get_some_data(a)} end) # => [%{1 => 123}, %{2 => 456}] How can I flatten the list of maps - res1 - so that I have this:
%{1 => 123, 2 => 456} While both Enum.map/2 and Kernel.SpecialForms.for/1 perfectly do their job, there is also a possibility to achieve the same result with Enum.reduce/3:
iex|1 ▶ input = [%{id: 1, data: :foo}, %{id: 2, data: :bar}] #⇒ [%{data: :foo, id: 1}, %{data: :bar, id: 2}] iex|2 ▶ Enum.reduce(input, %{}, fn e, acc -> ...|2 ▷ Map.put(acc, e.id, e.data) ...|2 ▷ end) #⇒ %{1 => :foo, 2 => :bar} 
Easiest thing here would be to construct a Keyword list during the map:
coll |> Enum.map(fn a -> {a.id, get_some_data(a)} end) |> Map.new Or:
for a <- coll, id = a.id, data = get_some_data(a), into: %{} do {id, data} end The problem was that you were building a map on each iteration, rather than building a key/value pair, which is what Keyword lists/maps are made of.
Enum.map_reduce won't help?
for the list construction is skipped entirely and the map is built directly, so that approach is more optimal.