Hi EEP list!
Here's yet another EEP, as requested by the OTP team in this comment:
https://github.com/erlang/otp/pull/4831#issuecomment-849615672
This one's isn't as controversial as some of the other ones (I hope).
Abstract
========
This EEP adds functions operating on lists of maps, analogous to the
functions `lists:key*` operating on lists of tuples, in a new module
in stdlib.
Read more in the attached EEP.
Thanks,
Viktor
Author: Viktor Söderqvist <viktor(dot)zuiderkwast(at)se>
Status: Draft
Type: Standards Track
Created: 18-Jun-2021
Erlang-Version: 25.0
Post-History: 18-Jun-2021
****
EEP XXX: Module for lists of maps
----
Abstract
========
This EEP adds functions operating on lists of maps, analogous to the
functions `lists:key*` operating on lists of tuples, in a new module
in stdlib.
Specification
=============
There shall be a module `maplists` in stdlib with functions analoguous
to the `lists:key*` and `lists:ukey*` functions, operating on lists of
maps. In place of the `N` argument (referring to key position in the
tuples), the `maplists` functions take a map key instead. The module
shall contain analogues of all the functions in the `lists` module
with a name containing "key", except `keysearch/3` (which is retained
only for backward compatibility). The function names shall be the
same as of those in the `lists` module. The complete list of
functions is:
* keydelete/3
* keyfind/3
* keymap/3
* keymember/3
* keymerge/3
* keyreplace/4
* keysort/2
* keystore/4
* keytake/3
* ukeymerge/3
* ukeysort/2
Each function in the `maplists` module shall ignore list elements
which are not maps, or which are maps but don't contain the requested
key, if and only if the corresponding function in the `lists` module
ignores list elements which are not tuples. Likewise, a function in
`maplists` which needs to compare all elements (namely `keymerge/3`,
`keysort/2`, `ukeymerge/3` and `ukeysort/2`) shall crash when list
elements other than maps or maps which don't contain the requested key
are present, if and only if the corresponding function in the lists
module crashes when the list contains elements which are not tuples.
Motivation
==========
One of the motivations for adding maps to Erlang was to "be a
complement to records and supersede them where suitable" ([EEP 43][]).
Maps and records both have their pros and cons and time has shown that
both are being used side by side in old and new code bases.
Stdlib's `lists` module has some functions operating on lists of
tuples, typically records, whose given element position matches a
given value. The functions for working with lists of tuples have
proven useful, but there aren't any functions for working with lists
of maps in the same way.
Such functions have been suggested from time to time, first in the
erlang-questions mailing list thread ["[erlang-questions] analogue of
lists:keyfind and other lists:key\*\*\* functions for lists which
contain maps"][2014] in 2014 (soon after maps were introduced) and
then again in 2019 ["[erlang-questions] lists module functions for
maps?"][2019]. Recently, the topic has come up in the OTP pull
request ["Add new lists:mapkeyfind/3 function to map finding
\#4831"][OTP-PR 4831].
Rationale
=========
The function names and the order of arguments are picked to match
those of the list-of-tuples functions as closely as possible. The
decision for functions to ignore list elements which are not maps is
also made to resemble the behaviour of the list-of-tuples functions.
This consistency allows for working with lists of maps and lists of
records in a similar way and to replace records with maps with the
least surprise possible.
The module name `maplists` follows a similar naming style as
`proplists` (lists of properties) and is in plural like `lists`,
`maps` and others in stdlib.
Reference Implementation
========================
[Lists of Maps][lom], by Craig Everett, has most of the functions in
this EEP, with the following differences:
* The module name is different.
* The function names don't have the "key" prefix.
* Most (if not all) of the functions in this implementation crash when
a list element which is not a map is encountered. Likewise, if a
map is lacking the requested key.
* The functions `ukeymerge/3` and `ukeysort/2` are missing.
[EEP 43]: eep-0043.md
"EEP 43, Maps, Björn-Egil Dahlberg"
[2014]: https://erlang.org/pipermail/erlang-questions/2014-November/081865.html
"[erlang-questions] analogue of lists:keyfind and other
lists:key*** functions for lists which contain maps"
[2019]: https://erlang.org/pipermail/erlang-questions/2019-September/098461.html
"[erlang-questions] lists module functions for maps?"
[OTP-PR 4831]: https://github.com/erlang/otp/pull/4831
"Add new lists:mapkeyfind/3 function to map finding #4831"
[lom]: https://gitlab.com/zxq9/lom/
"Lists of Maps"
Copyright
=========
This document is placed in the public domain or under the CC0-1.0-Universal
license, whichever is more permissive.
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