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 <jose.va...@dashbit.co> 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 <gvau...@gmail.com> 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 <jose.va...@dashbit.co> 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 <gvau...@gmail.com> 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 <jose.va...@dashbit.co> 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 <gvau...@gmail.com> 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 <allen.c.mad...@gmail.com> > > > > 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 <bjorn...@gmail.com> > > > > 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 <jose...@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 <gva...@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 > > > > > > > > > > -- > > > > > You received this message because you are subscribed to the Google > > > > > Groups "elixir-lang-core" group. > > > > > To unsubscribe from this group and stop receiving emails from it, > > > > > send an email to elixir-l...@googlegroups.com. > > > > > To view this discussion on the web visit > > > > > https://groups.google.com/d/msgid/elixir-lang-core/6B6AB775-F3D5-40E5-BFBD-9852FBCBD1D0%40gmail.com. > > > > > > > > > > > > > > > -- > > > > > You received this message because you are subscribed to the Google > > > > > Groups "elixir-lang-core" group. > > > > > To unsubscribe from this group and stop receiving emails from it, > > > > > send an email to elixir-l...@googlegroups.com. > > > > > To view this discussion on the web visit > > > > > https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KZPZ5mpP6SSzhmq3jpuZBYA1irpmOa19UNH2fS_3QKQA%40mail.gmail.com. > > > > > > > > > > > > > > > > > -- > > > > You received this message because you are subscribed to the Google > > > > Groups "elixir-lang-core" group. > > > > To unsubscribe from this group and stop receiving emails from it, send > > > > an email to elixir-lang-core+unsubscr...@googlegroups.com. > > > > To view this discussion on the web visit > > > > https://groups.google.com/d/msgid/elixir-lang-core/1ae0b9d3-9471-4750-8734-281033e9a1dc%40googlegroups.com. > > > > > > > > -- > > > > You received this message because you are subscribed to the Google > > > > Groups "elixir-lang-core" group. > > > > To unsubscribe from this group and stop receiving emails from it, send > > > > an email to elixir-lang-core+unsubscr...@googlegroups.com. > > > > To view this discussion on the web visit > > > > https://groups.google.com/d/msgid/elixir-lang-core/CAK-y3Cu%2BGBO1RWsdAjAHoaukV3w4QJPPdqqNU_miQ_%3Dv5%3DdDeQ%40mail.gmail.com. > > > > > > -- > > > You received this message because you are subscribed to the Google Groups > > > "elixir-lang-core" group. > > > To unsubscribe from this group and stop receiving emails from it, send an > > > email to elixir-lang-core+unsubscr...@googlegroups.com. > > > To view this discussion on the web visit > > > https://groups.google.com/d/msgid/elixir-lang-core/22988265-AB94-4666-894B-9ECF7B87905D%40gmail.com. > > > > > > -- > > > You received this message because you are subscribed to the Google Groups > > > "elixir-lang-core" group. > > > To unsubscribe from this group and stop receiving emails from it, send an > > > email to elixir-lang-core+unsubscr...@googlegroups.com. > > > To view this discussion on the web visit > > > https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4%2B5ovo9YdQHQO2m6i%3DL_SxPKRN4O4fZejH%3DXMXfJWwWkQ%40mail.gmail.com. > > > > -- > > You received this message because you are subscribed to the Google Groups > > "elixir-lang-core" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to elixir-lang-core+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/elixir-lang-core/4CE0D4F8-A341-4832-AC94-BDBC0D7E0911%40gmail.com. > > > > -- > > You received this message because you are subscribed to the Google Groups > > "elixir-lang-core" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to elixir-lang-core+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4K-yXsZ2mxJ3sg6knRwLAFmMUky6c0G50gaBVnDpb18fA%40mail.gmail.com. > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-lang-core+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/E9C59B12-1663-45E9-A8C3-163F75895D52%40gmail.com. > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-lang-core+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KwMg%3DHDM-B4B2Qh1irafG8_Y8tW%2BLYRTKGDoy%3DHOtkNA%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. 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