# [erlang-questions] Maps branch and disclaimers

Ivan Carmenates García co7eb@REDACTED
Sat Oct 26 16:43:15 CEST 2013

Hi all,

M0 = #{ key => Value1, "key" => Value2}, % for construction.
M1 = M1#{  "key" := Value3, <<"key">> => Value4 }, % for updates
#{ "key" := V } = M1. % for matching

I understand maybe the introduction of more operators like => and := for
construct the maps. But it does not make thinks a little more complex and
confusing?

Records have a similar structure like this:

R0 = #person{name = “Juan”, age = 29}

Maybe in the future you would like to implement anonymous records like this:

R1 = #{name = “Juan”, age = 29}

That’s why the operator = gets out of the question, but it is more easy to
learn the entire language when you don’t have so many different ways of
doing things.

Maybe something like this:

M0 = #{ key := Value1, "key" := Value2}, % for construction.
M1 = M1#{  "key" := Value3, <<"key">> := Value4 }, % for updates
#{ "key" := V } = M1. % for matching

Using the same operator for construction and update and matching, or even so
you can use the = operator, and when passing the term to maps does not
matter if for

some other module and its functions it is an anonymous record and for maps
that anonymous record means a map construction.

Ie:

M0 = #{ key = Value1, "key" = Value2}, % for construction.   What could be
in a future an anonymous record!
M1 = M1#{  "key" = Value3, <<"key">> = Value4 }, % for updates,

#{ "key" = V } = M1. % for matching

And all records like constructions will seems close equal.

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.

Best regards,

Ivan.

De: erlang-questions-bounces@REDACTED
[mailto:erlang-questions-bounces@REDACTED] En nombre de Klas Johansson
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 <egil@REDACTED>
wrote:

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.

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

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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