[erlang-questions] Maps branch and disclaimers

Pierre Fenoll <>
Sat Oct 26 22:46:27 CEST 2013


Hey Björn,

Looking at the grammar changes in erl_parse.yrl it seems that this
operation is not valid:

 M2 = fcall()#{  "key" = Value3, <<"key">> = Value4 }, % for updates,

"M1" has been changed to "fcall()", representing a (any) function call.
What that means is that one can't query maps directly from a function's
output. It just would not compile.

This issue can be fixed by just s/expr_max/expr_700/ on this line:
https://github.com/erlang/otp/blob/egil/maps/eep-implementation/lib/stdlib/src/erl_parse.yrl#L341
(Grammar rule expr_700 holds the function_call rule).
Maybe adding explicitly the function_call rule to the map_expr rule is
better though:

  map_expr -> …
  map_expr -> function_call '#' map_tuple : {map, ?line('$2'),'$1','$3'}.


Cheers,
-- 
Pierre Fenoll



On 26 October 2013 18:35, Björn-Egil Dahlberg
<>wrote:

>
>
>
> 2013/10/26 Ivan Carmenates García <>
>
>>
>>
>> You made distinguish a construction of an update by using of the M1#, but
>> I think that breaks the conviction of the Erlang language of immutable
>> variables, I know it is a way to update the records and it is only
>> syntactically but still that, it is ugly to see it syntactically. When you
>> never can do in a normal code A = A + 1, then you see it in maps updates. I
>> rather better something like this:****
>>
>> ** **
>>
>>   M2 = M1#{  "key" = Value3, <<"key">> = Value4 }, % for updates,****
>>
>> ** **
>>
>> When M2 is a new variable that will take the value of the current M1
>> variable with the modifications.
>>
>
> As I explained in an earlier answer, It was an unfortunate typo, it should
> have been,
>
>   M2 = M1#{  "key" = Value3, <<"key">> = Value4 }, % for updates,
>
>
> As for the two operators, it is fairly covered in the EEP on why and how.
> There are two because it removes the possibility of adding new keys where
> none should have been added, it is guarded by := , that's why.
>
> As for using the = operator, it is also covered in the EEP.
>
> // Björn-Egil
>
>
>
>
>> ****
>>
>>
>>
>> ****
>>
>> Best regards,****
>>
>> Ivan.****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> *De:*  [mailto:
>> ] *En nombre de *Klas Johansson
>> *Enviado el:* sábado, 26 de octubre de 2013 9:47
>> *Para:* Björn-Egil Dahlberg
>> *CC:* Erlang
>> *Asunto:* Re: [erlang-questions] Maps branch and disclaimers****
>>
>> ** **
>>
>> Hi,****
>>
>> ** **
>>
>> Nice!****
>>
>> ** **
>>
>> I noticed that syntax_tools needs to be updated with support for maps.
>>  Perhaps you already have that on some list?****
>>
>> ** **
>>
>> Keep up the good work. :-)****
>>
>> ** **
>>
>> ** **
>>
>> Cheers,****
>>
>> Klas****
>>
>> ** **
>>
>> ** **
>>
>> On Fri, Oct 25, 2013 at 6:37 PM, 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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>> 
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>>
>> ** **
>>
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