[erlang-questions] Maps branch and disclaimers

Tristan Sloughter tristan.sloughter@REDACTED
Sun Oct 27 01:55:43 CEST 2013

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:

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:


Thanks for maps!

On Fri, Oct 25, 2013 at 11:37 AM, Björn-Egil Dahlberg
<[3]egil@REDACTED> wrote:


Here you go, Maps!

I've pushed a Maps branch to Erlang/OTPs repository at GitHub.

To get the branch,

  git fetch [4]git@REDACTED:erlang/otp.git

or find it at

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
[6]http://github.com/erlang/eep/blob/master/eeps/eep-0043.md, for

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.


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

    =:=         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

Happy testing!


Björn-Egil Dahlberg



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4. mailto:git@REDACTED:erlang/otp.git
5. https://github.com/erlang/otp/tree/egil/maps/eep-implementation
6. http://github.com/erlang/eep/blob/master/eeps/eep-0043.md
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