Thanks, José. I agree with the need to be consistent. I will look at the bigger
picture, though, like Manfred I find the addition of "maybe" to be awkward, so
my preference is to have the existing recommended functions in the Access
module intended for use with get_in to be consistently nil safe. I'm open to
more ideas, too.
-Greg Vaughn
> On Jan 28, 2020, at 12:45 PM, José Valim <jose.va...@dashbit.co> wrote:
>
> The proposal is reasonable however it would introduce an inconsistency since
> the other selectors in Access, such as Access.key, are also not nil safe. So
> whatever solution we choose needs to be consistent.
>
> One possible suggestion is to introduce a "Access.maybe" that composes but
> composition would have to be back to front:
>
> %{"items" => nil} |> get_in(["items", Access.at(0) |> Access.maybe])
>
> Another idea is to introduce maybe_at, maybe_key, maybe_key! and so on. But I
> am not sure if this is desirable. Thoughts?
>
> On Tue, Jan 28, 2020 at 7:33 PM Greg Vaughn <gvau...@gmail.com> wrote:
> I propose that the function returned from Access.at/1 special case nil such
> that the overall Kernel.get_in/2 call returns nil instead of raising an error.
>
> Rationale:
> I originally blamed this on Kernel.get_in/2 and I'd like to thank Eric
> Meadows-Jönsson for explaining the underlying reason to me on Slack.
>
> I like to think of Kernel.get_in/2 as a nil-safe way of plucking values out
> of nested data structures, but I learned today that is only partially
> correct. The nil-safety comes from the underlying Access.get/2 calls. The
> docs for get_in includes:
>
> In case any of the entries in the middle returns nil, nil will be returned
> as per the Access module:
> iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
> iex> get_in(users, ["unknown", :age])
> nil
>
> and I expected use of Access.at/1 in my keys to act similarly, but it
> doesn't. For example:
>
> iex(185)> %{"items" => ["desired_value"]} |> get_in(["items", Access.at(0)])
> "desired_value"
> iex(186)> %{"items" => nil} |> get_in(["items", Access.at(0)])
> ** (RuntimeError) Access.at/1 expected a list, got: nil
> (elixir) lib/access.ex:663: Access.at/4
>
> I propose that the function returned from Access.at/1 special case nil such
> that the overall get_in/2 call returns nil instead of raising an error. I
> have not dug into the source yet but I'm happy to work up a PR if there is
> interest in this change.
>
> -Greg Vaughn
>
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