[erlang-questions] Maps branch and disclaimers

[Björn-Egil Dahlberg]

2013/10/28 Robert Virding <robert.virding@REDACTED>

> Great! But I do have some comments (of course):
>

Great! =)


>
> - I would keep the function names from the dict module where possible.
> This to make it easier to convert.
>

I agree with your point. Ease of conversion would be great. But, why would
dict be more suitable than gb_trees ? The current API is of personal
preference ofc.



>
> - I definitely agree about creating true term ordering comparison
> operators to complement =:= and =/=. But as I am more used to the prolog
> set (@==, @/=, @>, @>=, @< and @=<) I would much prefer to have them
> instead. They at least start with the same character. Anyway create the
> operators and define maps to use them, and then fix dict/orddict/gb_trees
> to use them as well so they are compatible. I could survive potential
> backwards compatibility problems with them to achieve consistency.
>

I'm thinking of writing another EEP. I did a test implementation of <:<,
>:>, =:<, >:= this weekend to see how it would pan out. See,
https://github.com/psyeugenic/otp/commits/egil/total-order-relops for
details. Seemed ok. The syntax can be changed to suite prolog heritage but
the @... syntax freak me out to be honest =)

Also what should we do with =:= vs @== and =/= vs @/= in that case? Should
we have two operators with the same meaning?


>
> - I also definitely agree to keep the syntactic footprint to a minimum and
> introduce as little new syntax as possible.
>

Check.


>
> - For map comprehensions I would NOT use <- as the generator operator as
> it tells me we are generating elements from a list. Binaries got a new
> operator and I think maps should as well, for example <:, or something else.
>

There are currently no generator implemented, and it might not be
implemented either. Also, I think K := V <- Map works for the visual and it
is definitely unique enough to be parsable. The difference of #{ K := V }
<- List vs. K := V <- Map might be to subtle though. I think we might have
to revisit this and give maps its own arrow.


Thank you!

//Björn-Egil


>
> Robert
>
>
> ------------------------------
>
> *From: *"Björn-Egil Dahlberg" <egil@REDACTED>
>
>
> Hi!
>
> Here you go, Maps!
>
> I've pushed a Maps branch to Erlang/OTPs repository at GitHub.
>
> To get the branch,
>
>   git fetch git@REDACTED:erlang/otp.git egil/maps/eep-implementation
>
> or find it at
> https://github.com/erlang/otp/tree/egil/maps/eep-implementation
>
> I want to state the following so there is no room for uncertainty:
> - This branch contains a *development stage* of the *experimental* Maps
> feature for Erlang.
>
> This means:
>  - Do not use it in production since it is not stable,
>  - Do not base any git branch on this branch since it will most likely be
> rebased,
>  - and finally, we reserve the right to change any API or interfaces to
> Maps currently implemented.
>
> The implementation is based on EEP 43 - Maps, see
> http://github.com/erlang/eep/blob/master/eeps/eep-0043.md, for details.
>
> What is implemented?
>
> The maps module API and erlang guard BIFs as defined in the EEP are
> implemented. There are however some sematic mismatches with the EEP. I
> think those are where the definition contradict itself. For instance
> maps:is_key/1 compares with =:= as stated first in the definition but the
> later example uses lists:keymember which compares with ==.
>
> The syntax and all what that entails is implemented. The compiler will
> handle the map syntax and produce loadable beam-code. I believe this is
> what people want to test and is what I want people to test. Test the
> usability that is.
>
> I recommend people look at the EEP for information and also the testsuite
> located at erts/emulator/test/map_SUITE.erl for information on how to use
> Maps since no other documentation is available.
>
> Roughly,
>   M0 = #{ key => Value1, "key" => Value2}, % for construction.
>   M1 = M1#{  "key" := Value3, <<"key">> => Value4 }, % for updates
>   #{ "key" := V } = M1. % for matching
>
> Where the operator '=>' (assoc operator) is used for extending and
> creating new Maps and the operator ':=' is used to update existing
> key/values. The ':=' operator is the only operator allowed in patterns. I'm
> guessing some confusion will arise from these two types of operators on
> where you can and/or should use them.
>
> Look at the tests and EEP for details and inspiration.
>
> A major difference from the EEP are variables in keys. Variables in keys
> are not allowed at all. This is because we want to reduce the scope for
> this first stage. Plenty to do besides that.
>
> Here are some additional disclaimers to make people sad.
>
> What is not implemented?
>
> - No variable keys.
> - No single value access.
> - No map comprehensions.
> - No datastructure to handle large Maps.
> - No MatchSpecs which uses the Maps syntax will work.
>
> Known issues
>
> - Dialyzer will not work with maps in the code, this include PLT building
> with erts and stdlib.
> - HiPE, the native compiler, will not with maps code.
> - EDoc will not work with maps.
>
> I'm sure there are other issues as well, it is a development branch after
> all. =)
>
> I would also like to point out that no optimizations are done either with
> respect to the generated code. This means that the instruction set may
> change. We know of several optimization we want to do before R17,
> especially for the match compiler so keep that in mind.
>
> We will continue stabilizing the Maps implementation as we move forward
> towards R17 and take appropriate action depending on the feedback you give
> us.
>
> I would like to continue with saying a few words about possible changes
> that we are thinking about.
>
> Variables in Keys
>
> This feature is actually furthest down on the work prio list. We want to
> stabilize the current features before moving forward and variable keys is
> the one most likely to be dropped if we get pressed for time. Meaning, it
> might not be implemented for R17 but instead implemented for R18. The plan
> right now is to keep it though.
>
> The External Format
>
> The current external format *needs* ordered keys as input for
> binary_to_term/1 and in distribution.
>
> This is of course an inconvinience when dealing with other language
> interfaces which has no idea of what the erlang term order is. I instead
> propose that the external format should handle unordered input of key-value
> pairs. The trade off is a more complicated decoding which will take longer.
>
> The distribution format should also be extended to be able cache keys.
> This is similar to the atom cache except we
> cache the entire key array for maps. This has been the intention all along
> but it not mentioned in the EEP.
>
> Term order and sorting
>
> Finally the term order. This has been a sore point from the get go.
>
> Maps currently respects the Erlang term order for it's keys.
>
> The Erlang term order is what I call arithmetic term order. I propose that
> we extend Erlang with true term order where integer compares less then
> float, i.e. total term order.
>
> This would allowing newer ordered data structures, like maps, to be more
> useful. We don't have to take
> special care for the odd cases like keys 1.0 and 1 inhabiting the same
> slot in the data structure. gb_trees and such structures could also be
> extended to use this as those structures has the same limitations.
>
> With this type ordering we could have maps with this type of keys, #{ 1 =>
> "integer", 1.0 => "float" } without causing confusion.
>
> I've been told that ETS ordered sets tables used to have this behaviour.
> Distinguishing between floats and integers. This was supposedly before the
> open source era, way back when dinosaurs roamed the planet .. I'm not clear
> on the details on why this behaviour was removed. Probably because of
> inconsistencies.
>
> For maps to work with this I only need two things. First, a compare
> operation in the runtime that can distinguish between floats and integers,
> very easy. Secondly, a BIF that sort a list of terms with this new compare
> operation which will be used in the compiler.
>
> But for completness, the following operators should also be implemented:
>
>     =:=         term exact equal to, already implemented
>     =/=         term not equal to, already implemented
>     =:<         term less or equal than
>     >:=         term greater or equal than
>     <:<         term less than
>     >:>         term greater than
>
> So, true = 1 <:< 1.0.
>
> I don't know prolog but perhaps these sematics should mimic prolog to
> respect Erlangs heritage. I have no strong opinion on this.
>
> This syntax would mimic the already present =:= and =/= relational
> operators hower this syntax is another topic and should be a seperate EEP.
>
> Happy testing!
>
> Regards,
> Björn-Egil Dahlberg
> Erlang/OTP
>
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