[erlang-questions] Maps branch and disclaimers

Anthony Ramine <>
Sun Oct 27 20:09:23 CET 2013


Abstract modules don’t exist. The leftover from pmods is a really ugly hack where you can do an external call with a tuple as a module.

As for my thoughts: no.

What is the point of such an indirection? Why do you need to abstract over multiple implementations? Choose an adequate data structure for your domain problem and stick with it or change it if you find a better implementation.

Regards,

-- 
Anthony Ramine

Le 27 oct. 2013 à 11:58, Gleb Peregud <> a écrit :

> Great work, OTP team!
> 
> I was wondering about one thing. I find abstract modules (a leftover
> of parametrized modules) a very neat feature in some cases and I was
> wondering if anyone else would like to be able to something like this
> with maps:
> 
> M0 = #{ key => Value1, "key" => Value2}, % for construction.
> M1 = M0:put("key", Value3), % mapped to maps:put("key", Value3, M0)
> M2 = M1:put(<<"key">>, Value4)
> 
> Do you think this is a desirable feature? It can be useful for
> creating some kind of abstraction layer over multiple implementations
> of the same interface without having to use macros nor implementing a
> proxy modules to handle this.
> 
> Thoughts?
> 
> Cheers,
> Gleb
> 
> On Sun, Oct 27, 2013 at 1:55 AM, Tristan Sloughter
> <> wrote:
>> This is great! And fun. Waiting so long to play with these I had to make a
>> branch of jsx using maps for objects today:
>> https://github.com/tsloughter/jsx/tree/maps
>> 
>> --
>> Tristan Sloughter
>> 
>> 
>> 
>> 
>> On Fri, Oct 25, 2013, at 07:22 PM, Daniel Goertzen wrote:
>> 
>> Works like a charm!
>> 
>> Yamler (Erlang yaml file loader) now constructs yaml mappings as erlang
>> maps.  I updated the README examples to show these maps in action:
>> 
>> https://github.com/goertzenator/yamler/tree/mapping_as_map
>> 
>> 
>> Thanks for maps!
>> Dan.
>> 
>> 
>> On Fri, Oct 25, 2013 at 11:37 AM, Björn-Egil Dahlberg <>
>> wrote:
>> 
>> 
>> Hi!
>> 
>> Here you go, Maps!
>> 
>> I've pushed a Maps branch to Erlang/OTPs repository at GitHub.
>> 
>> To get the branch,
>> 
>>  git fetch :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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