I'm uncertain how best to proceed at this point. I have some code that is a
proof of concept of what I believe is a compromise. I don't wish to open
another PR prematurely and have it closed, so I'll try one more round of
discussion here.
I have added 2 new Access functions: path/1 and path!/1. The first one is nil
safe with a wrapper as José mentioned earlier. The second one offers consistent
assertiveness, not the hybrid behavior of get_in today. Here's the doctests I
started with for further discussion.
for path/1
iex> get_in(%{}, Access.path([:a, :b]))
nil
iex> get_in(%{}, Access.path([:a, Access.at(0)]))
nil
iex> get_in(%{a: nil}, Access.path([:a, Access.at(0)]))
nil
iex> get_in(%{a: []}, Access.path([:a, Access.at(0)]))
nil
for path!/1
iex> get_in(%{}, Access.path!([:a, :b]))
** (KeyError) key :a not found in: %{}
iex> get_in(%{}, Access.path!([:a, Access.at(0)]))
** (KeyError) key :a not found in: %{}
iex> get_in(%{a: nil}, Access.path!([:a, Access.at(0)]))
** (ArgumentError) Access.path!/1 encountered nil
This one is unimplemented yet, but it should raise for consistency:
iex> get_in(%{a: []}, Access.path!([:a, Access.at(0)]))
** (ArgumentError) [] has no element at index 0
For comparison, this is how get_in behaves with these cases today, highlighting
the inconsistency that is the core of what bugs me. Half the cases return nil
and half raise.
iex> get_in(%{}, [:a, :b])
nil
iex> get_in(%{}, [:a, Access.at(0)])
** (RuntimeError) Access.at/1 expected a list, got: nil
iex> get_in(%{a: nil}, [:a, Access.at(0)])
** (RuntimeError) Access.at/1 expected a list, got: nil
iex> get_in(%{a: []}, [:a, Access.at(0)])
nil
If this is a welcome direction, I'll be happy to submit my PR and work through
details of optimization, cleanup, exception wording, etc.
-Greg Vaughn
> On Feb 8, 2020, at 1:47 AM, José Valim <[email protected]> wrote:
>
> For now, I don't think we should add a new function to Kernel. So we should
> find something that makes a path nillable for definition in Access, and then
> you can define get_path in your app if that's what you prefer.
>
> On Sat, Feb 8, 2020 at 2:23 AM Greg Vaughn <[email protected]> wrote:
> I recognize more use cases than mine. Given we will not change Kernel.get_in,
> I have ideas for other, less "nillable" names, such as "get_path" or
> "path_in" to make mil-safety less of an exceptional situation. Path
> expressions, as originally used in object oriented databases, typically did
> not raise exceptions when some data did not match expectations. This specific
> naming discussion can be deferred though.
>
> I am in agreement on writing assertive code. That is the very reason I want
> something in the standard library that is a nil-safe navigation through
> untrusted input. I don't want to write an `if` or `with` dealing with each
> list key that might be nil, when I don't have to do it for maps. It it is the
> very reason I view the dot syntax as very confident keys exist vs. a get_in
> call which uses Access to determine existence of keys/lists.
>
> -Greg Vaughn
>
> > On Feb 7, 2020, at 6:55 PM, José Valim <[email protected]> wrote:
> >
> > > What I find curious is that once we implement Kernel.nillable_get_in, why
> > > would anyone choose to use Kernel.get_in instead?
> >
> > When I don't expect anything to be nil, I want it to fail as soon as
> > possible, instead of having nil further creeping into the system.
> > Personally, most of the times I used get_in and friends, I am working with
> > structured data (the opposite of your use case). If any nil shows up, it
> > should be an error.
> >
> > And changing get_in may not break code, expectations I had when I wrote the
> > code would certainly be broken. And I would personally be unhappy if we
> > simply changed get_in without introducing an option to write assertive
> > code. Writing assertive code is an important of Elixir. It is why we have
> > map.foo in addition to map[:foo]. So I think it is best to remove changing
> > get_in from the discussion altogether, I don't see it happening.
> >
> > We can continue discussing alternatives though.
> >
> >
> >
> > On Sat, Feb 8, 2020 at 1:47 AM Greg Vaughn <[email protected]> wrote:
> > One more point. Even if my proposal is not accepted, these docs for
> > Kernel.get_in really need to change
> >
> > 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
> >
> > The Access module guarantees no nil-safety. It's an "accident" that
> > Access.get does.
> >
> > -Greg Vaughn
> >
> >
> > > On Feb 7, 2020, at 4:40 PM, José Valim <[email protected]> wrote:
> > >
> > > Hi Greg, I have been thinking more about this too, and I think there are
> > > some neat ways we can make this more accessible:
> > >
> > > We could introduce Access.nillable (please suggest a better name) that
> > > you would use like this:
> > >
> > > get_in(root, Access.nillable([:foo, :bar, Access.at(0)]))
> > >
> > > Basically, it traverses the path and sets all functions in the path to
> > > something that handles nil. In your apps, you can quickly encapsulate it
> > > like this:
> > >
> > > nillable_get_in(root, [:foo, :bar, Access.at(0)])
> > >
> > > It is concise, backwards compatible, and clear in intent.
> > >
> > > Thoughts?
> > >
> > > On Fri, Feb 7, 2020 at 11:35 PM Greg Vaughn <[email protected]> wrote:
> > > I just wanted to follow up and summarize here. I submitted a PR
> > > https://github.com/elixir-lang/elixir/pull/9773 with some more
> > > discussion, but the core point there was that we needed more discussion
> > > on the core list before a PR and it was closed. Nil-safety by default is
> > > undesirable in more Access functions than Access.get.
> > >
> > > I'm exploring this on my own in my own codebase as I rework all the
> > > get_in calls I assumed were nil safe despite using Access.at. I am quite
> > > against a solution that is more verbose to gain mil safety as I use this
> > > at the edges of my system in an anti-corruption-layer. I'd rather see
> > > this implemented once, well, in the standard library than expect
> > > thousands of projects to do it themselves or bring in a 3rd party
> > > solutions to achieve it.
> > >
> > > Feel free to discuss some more.
> > >
> > > -Greg
> > >
> > > > On Jan 30, 2020, at 12:02 PM, Allen Madsen <[email protected]>
> > > > wrote:
> > > >
> > > > I'm in favor of them being nilsafe by default.
> > > >
> > > > Allen Madsen
> > > > http://www.allenmadsen.com
> > > >
> > > >
> > > > On Wed, Jan 29, 2020 at 11:24 AM Tor Bjornrud <[email protected]>
> > > > wrote:
> > > > I wouldn't mind having opts for something like this. Avoids creating a
> > > > slew of Access functions that then become difficult to sift through.
> > > >
> > > > %{"items" => nil} |> get_in(["items", Access.at(0, nilsafe: true)
> > > >
> > > > On Tuesday, January 28, 2020 at 8:10:04 PM UTC-6, Greg Vaughn wrote:
> > > > 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 <[email protected]> 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 <[email protected]> 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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