`Map.get/3` to return default value for `nil` value

[Nganga Mburu]

I've found myself and seen others expecting to receive the default value from `Map.get/3` when the value is `nil`.

An example is:

    `Map.get(%{name: nil}, :name, "John")` 

    

     Expected it to return "John" but it returns `nil`

My thinking is since the default value is for the key then when the key value is `nil` we should be able to get the default value.

The change in code could be as below:

     ```

     def get(map, key, default \\ nil) do

        case map do

           %{^key => value} when is_nil(value) ->

               default

           %{^key => value} ->

                value

           %{} ->

               default

           other ->

               :erlang.error({:badmap, other}, [map, key, default])

        end

     end

     ```

[Andrey Yugai]

For this behavior I think you'd be better of using `map[key] || default_value`, or even `Map.get(map, key) || default_value`. Docs for `Map.get/3` are pretty clear about when it returns default value.

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[Nganga Mburu]

We've used `map[key] || default_value`, or even `Map.get(map, key) || default_value` before but I felt this being handled my `Map.get/3` would be better.

`Map.get(map, key, default_value)` looks better than `Map.get(map, key) || default_value`

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[Andrey Yugai]

It indeed looks nicer, but that would be a breaking change, and maybe more importantly, subduing two different cases to one behavior. Sure, in your case you don't need to differentiate between an empty map and nil-instantiated map, but somebody else does. Perhaps you could change the upstream code to send you only nil key stripped maps or only fully instantied maps (e. g. structs).

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[Austin Ziegler]

On Jun 22, 2022, at 04:37, Nganga Mburu <mburun...@gmail.com> wrote:



We've used `map[key] || default_value`, or even `Map.get(map, key) || default_value` before but I felt this being handled my `Map.get/3` would be better.

`Map.get(map, key, default_value)` looks better than `Map.get(map, key) || default_value`

This assumes that `nil` isn’t a valid value. For your use-case it may be. For other cases, it may be a valid and important value.

I know for a fact that this would break some production code we run and be a surprising result for us because it would not behave the way we expect for anything else, including JavaScript Map.get.

-a

[Ben Wilson]

> `Map.get(map, key, default_value)` looks better than `Map.get(map, key) || default_value`

This isn't a matter of looks, these have straight up different functionality. You can't change this sort of thing in the language at this point in its lifecycle. Plus, you'd be forcing everyone who's doing that first thing to change to the far uglier multiline:

```

case Map.fetch(map, key) do

  {:ok, value} -> value

  _ -> default_value

end

```

[Nganga Mburu]

Thanks a lot all for the responses. I get how this would break things considering `Map.get/3` is foundational to many other code bases. So we can consider this as better handled as it's currently done.

Thank you again.

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