Overall I like the idea of having a separate function that throws an error
in case there are duplicated keys. Just don't think it's the place to be
done on Map.new/2 It works that way on Erlang maps:from_list and also with
for
for n <- input, into: %{}, do: {n.name, n.age}
2017-10-03 23:23 GMT-04:00 Myron Marston <[email protected]>:
> Map.new/2 is quite useful and I use it frequently. Whenever I use it, I
> expect that my transform function returns unique keys, so that
> Enum.count(input)
> == map_size(output) holds true. Unfortunately, this hasn’t always held
> true for the data I’m working with, and it results in an output map that’s
> essentially “lost” some of the input data. Here’s an example:
>
> iex(2)> input = [%{name: "Bob", age: 37}, %{name: "Jill", age: 24}, %{name:
> "Bob", age: 0}]
> [%{age: 37, name: "Bob"}, %{age: 24, name: "Jill"}, %{age: 0, name: "Bob"}]
> iex(3)> Map.new(input, &{&1.name, &1.age})
> %{"Bob" => 0, "Jill" => 24}
>
> Here the %{name: "Bob", age: 37} data has been lost because it was
> overwritten by %{name: "Bob", age: 0} since maps obviously can’t have
> duplicates. In this situation, it would be really nice if Map.new/2 did
> not silently ignore the fact that I gave it duplicate keys. I’d like a
> strict from of Map.new/2 that raises an error when your transform
> function returns duplicate keys, to notify you that you aren’t using it
> properly.
>
> If we add this, I’m not sure if it can just be a change to Map.new/2
> (would that be a breaking change for some users?) or if we’d need to add an
> alternate function like Map.new!/2 or Map.strict_new/2 or something.
>
> Thoughts?
>
> Myron
>
>
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