[proposal] Consider supporting a map shorthand syntax

[Devin Torres]

Taking a pretty cool page out of ES6:

# If `method`, `url`, `headers`, and `payload` are already bound

%Request{method, url, headers, payload}

# If e.g. `payload` hasn't been bound yet

%Request{method, url, headers, payload: get_payload()}

[Aleksei Magusev]

It would be awesome to have that syntax; and for update as well.

I assume that Alexei Sholik also likes this idea: https://botbot.me/freenode/elixir-lang/2015-05-27/?msg=40248917&page=7

[Devin Torres]

Yeah, it's a very common pattern it feels like.

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[Robert Virding]

Couldn't that make the syntax get rather messy when you mix?

Robert

[Devin Torres]

I'm not sure. The ES6 code I've written has sometimes been mixed, and I don't think it's been messy so far--but I guess that's subjective.

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[Robert Virding]

Should the syntax allow writing

%Request{method, url: get_url(), headers, payload}

Robert

[Dmitry Aleksandrov]

May be it is really cool syntax, I see only one problem, that: variable name becomes data. Does it good? (Do not have opinion, only ask about it)

[Ramon Snir]

This is also what SML has: https://gist.github.com/psibi/4682181

[Vincent Siliakus]

Something like this has been discussed here before:

https://groups.google.com/forum/#!topic/elixir-lang-core/4w9eOeLvt-8/discussion

In that discussion the consensus was that all 'field puns' (as they are apparently called) should be placed before regular fields, so your example would be only valid as:

%Request{method, headers, payload, url: get_url()}

If we still want this feature, I strongly agree with the 'field puns first' restriction, as it makes code arguably easier to read, less prone to errors and probably also easier to implement.

-vincent

[Josh Adams]

Throughout the course of today I've had a bit of a change of heart on this.  I haven't used it personally, so maybe it is useful (and I know I type `thing: thing` in structs tons), but the fact that variable names become important definitely hits my "zomg-bone".  I worry that this feature makes code much harder to read and encourages a bit of magic that I don't much love.

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Josh Adams

[Peter Hamilton]

This is something fairly easily accomplished via macros, so we can use it for a bit without having to commit to it. I did something similar a while back with:

extract_vars_from_map(:foo, :bar, :baz, :qux) = params

was transformed to:

%{"foo" => foo, "bar" => bar, "baz" => baz, "qux" => qux} = params

It was fairly useful with Phoenix, as I was frequently unpacking all those values into variables.

Someone give it a shot, share the code and see if it's useful.

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[Andrea Leopardi]

I'm with Josh on this one. Variable names having meaning may be confusing in my opinion; yes it's often tedious to do `foo: foo` but I prefer clarity over conciseness in this case.

On Sunday, May 31, 2015 at 12:19:46 PM UTC+2, Devin Torres wrote:

[Drew Olson]

I don't like this. I prefer more explicitness even if it requires some more verbosity / name repetition in this case.

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[Josh Adams]

On Wed, Jun 3, 2015 at 4:09 AM, Andrea Leopardi <an.le...@gmail.com> wrote:

yes it's often tedious to do `foo: foo` but I prefer clarity over conciseness in this case.

The worst example of this in my experience was an Angular app where we had a bit of scope value called 'data' stored in a value called data.  Passing this in with  the 'data-' prefix to make it pass html attribute parsing led to:

<widget data-data=data"/>

So 2 isn't so bad :)

[Saša Jurić]

Assuming that %{foo, bar, baz, ...} is simply expanded into %{foo: foo, bar: bar, baz: baz, ...}, I like this proposal. I don't find it implicit nor magical, and I think it might make the code more readable in some cases.

I've occasionally experienced that foo: foo may add a lot of noise in functions which pattern match on maps, especially if I'm matching a couple of values and/or multiple maps. I think this shorter syntax might help with that.

On Sunday, May 31, 2015 at 12:19:46 PM UTC+2, Devin Torres wrote:

[Bruce Tate]

This is going to be a big deal for Phoenix. In our channels where we're matching against the message, we have lines like this pervasively:

def handle_in(event, %{"chat" => chat, "question_id" => question_id, "data" => data, "attachment" => attachment, ...}, socket) do...

def handle_in(event, %{chat, question_id, data, attachment, ...}, socket) do...

It's the same model in controllers, like this:

def delete_question(conn, %{"survey_id" => survey_id, "question_id" => question_id, ...}) do

def delete_question(conn, %{survey_id, question_id, ...}) do

My guess is that this pattern is going to show up over and over in production elixir, as business concerns have more detail that we're going to want to move into structs and maps. We'll have to work out some details, like atoms versus strings, but we can support both pretty easily.  

I strongly support this proposal. To solve this problem, we're already moving toward macros (to keep things short, we're going to use a sigil, but it's distasteful and we'd prefer an in-language alternative. We really need a shorthand way to handle this concern. 

We will volunteer to take a pass at this solution, if we agree that this is a path that is interesting. 

Thanks, 

Bruce Tate (with Eric looking over my shoulder as we speak). 

[José Valim]

This is going to be a big deal for Phoenix. In our channels where we're matching against the message, we have lines like this pervasively:

def handle_in(event, %{"chat" => chat, "question_id" => question_id, "data" => data, "attachment" => attachment, ...}, socket) do...

def handle_in(event, %{chat, question_id, data, attachment, ...}, socket) do...

It's the same model in controllers, like this:

def delete_question(conn, %{"survey_id" => survey_id, "question_id" => question_id, ...}) do

def delete_question(conn, %{survey_id, question_id, ...}) do

I have stayed mostly out of the discussion so I just want to point out that you are proposing something different than what was proposed so far. All the proposals were about %{survey_id, question_id} matching on *atom* keys. You are proposing to match on strings. And we need to pick one, we certainly cannot have the same syntax matching on both.

If we are sure we want both, then we need two syntaxes (and %{"survey_id", ...} doesn't cut it imo). We use sigils to handle a similar case, so we could adopt them here: 

    ~m(question_id survey_id)a #=> atom keys, the default

    ~m(question_id survey_id)s #=> string keys

But that is really up for everyone that have participated on this thread so far.

[Peter Hamilton]

I'm opposed to a special syntax in Elixir itself. Aside from the confusion around two different behaviors (as Jose pointed out), there's even a third semantic case for %{foo,bar,baz} and that's "the map contains the keys (no assignments)".

I'm in favor of the sigil approach, or something similar, in a library. First, make it possible for people to use the syntax and then make it easy to use (ie, language builtin) as we get feedback from the community.

Just my 2 cents.

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[Andrea Leopardi]

I stood against the %{foo, bar, baz} syntax and deemed that too implicit, but I'm in favour of a possible ~m sigil. I think it may still be a bit implicit, but I can see it would make code much more concise in a lot of situations. I took a stab at it and wrote a simple implementation for such sigil, here. We can try it out and see if we like it (and please, have a look at the implementation as well as I'm not sure it's bulletproof).

On Sunday, May 31, 2015 at 12:19:46 PM UTC+2, Devin Torres wrote:

[Eric Meadows-Jönsson]

Nice work Andrea. One small comment; I think it would be nicer to default to string keys so that we have the same defaults as the ~w sigil.

It would also be nice to still be able to match on structs to match Devin's original example, possibly: ~m(Request method url headers payload). I don't think there is a nice way, with sigils, to match bound and unbound variables as in the second example.

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[Andrea Leopardi]

Hey, thanks! Defaulting on strings makes sense to match the default behaviour of the ~w sigil. Yes, matching structs would be nice and the syntax you proposed is probably the best (and only) option, even if I'm not a fan of it. What we could do though is to at least provide a third modifier, S, which specifies that ~m is matching a struct instead of a map, so that we don't take capitalized keys off the table and it's clear that the whole thing is a struct.

~m(Request method url headers payload)aS

What do you think?

Yes I don't think with sigils matching both bound and unbound variables is possible, but at the same time I don't think that use case is as common as matching on a map or struct and assigning variables.

[Eric Meadows-Jönsson]

No extra modifier is required since there is no ambiguity for Request in ~w(Request foo bar). Variables cannot be capitalized so we can assume that Request is a module alias.

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[Andrea Leopardi]

Right! Completely oversaw that, sorry :) will fix soon.

Thanks for the feedback!

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[José Valim]

Yeah, I disagree with the struct syntax. :) just keep it simple.

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José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

[Andrea Leopardi]

José, do you mean you disagree with supporting structs at all in the ~m sigil or just with the syntax?

[Josh Adams]

I'm really happy with this.  I did see the duplicity as a problem but I thought the 'magic' would be confusing.  A sigil is an excellent compromise that brings all the goodness without the possible confusion.  :)

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[José Valim]

I meant supporting structs in the ~m syntax, even more if it will default to strings.

José Valim

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Founder and Director of R&D

[Johan Wärlander]

After playing around a bit with the ~m sigil, I proposed a couple of updates (one which touched on work that Andrea had alredy done, but with a twist); short_maps now support the following:

# Equality checks

foo = 1

bar = 2

~m(foo bar)a == %{foo: 1, bar: 2} #=> true

# Matching

~m(foo bar)a = %{foo: 12, bar: "baaz"}

foo #=> 12

# Pinning (NEW)

foo = 1

~m(^foo bar)a = %{foo: 1, bar: "baaz"} #=> %{foo: 1, bar: "baaz"}

~m(^foo bar)a = %{foo: 5, bar: "baaz"} #=> MatchError

# Structs, when first word starts with '%' (NEW)

defmodule Foo do

  defstruct bar: nil

end

~m(%Foo bar)a = %Foo{bar: "baaz"}

bar #=> "baaz"

For anyone else interested, please do get the latest version from Andrea's repository (https://github.com/whatyouhide/short_maps), then play around with it and see how it feels.

[Jaap Frolich]

Very nice Johan,

Love the power of macros. Is there any resolution if this will make the standard library? I have a lot of repetition in my maps as well in my current app, and this is one of the things I like in ES6 that I really miss in Elixir (mostly it is of course the other way around ;)).

Cheers,

Jaap

[Hassan Zamani]

I would appreciate hearing your thoughts on https://github.com/hzamani/synex. It has two macros: keys and params, first one for expansion of atom-value pairs in keyword lists, maps and structs, and other for expansion of string-value pairs in maps. Both of them have variable pinning and map update support and params supports nested maps:

iex> params(%{a => %{b, c}}) = %{"a" => %{"b" => 2, "c" => 3}}

iex> a

%{"b" => 2, "c" => 3}

iex> c

3

Hassan

[Hassan Zamani]

No comments?

[Ben Wilson]

You necro'ed a thread that was effectively a year old, it isn't an ideal place to receive feedback.

Maybe try posting on the elixir forum?

[OvermindDL1]

Looks interesting, but how does it compare to https://hex.pm/packages/short_maps as it adds a sigil that does something similar?

```elixir

import ShortMaps

my_map = %{foo: 1, bar: 2, baz: 3}

~m(foo bar baz)a = my_map
foo #=> 1

import ShortMaps

name = "Meg"

# String keys by default (or with the 's' modifier)
~m(name) #=> %{"name" => "Meg"}
# Atom keys with the 'a' modifier
~m(name)a #=> %{name: "Meg"}

iex(1)> import ShortMaps
nil
iex(2)> name = "Meg"
"Meg"
iex(3)> ~m(^name)a = %{name: "Meg"}
%{name: "Meg"}
iex(4)> ~m(^name)a = %{name: "Megan"}
** (MatchError) no match of right hand side value: %{name: "Megan"}

```

And other stuff

[Wiebe-Marten Wijnja]

Hmm... this is the kind of feature that I would've used a lot, had I known that a package that provided this shorthand syntax existed.

I will definitely do so in the future. I really like the sigil idea.

Are we still opposed to including the `~m` sigil in the base language?

[OvermindDL1]

If it were included I would want it to be a bit more useful (and could perhaps be rolled into the map syntax itself with sigil, but I'm fine with it as a library):  https://github.com/whatyouhide/short_maps/issues/13

[Hassan Zamani]

One of main differences of Synex and ShortMaps is that Synex supports nested stractures:

iex> params(%{a => %{b, c}}) = %{"a" => %{"b" => 2, "c" => 3}} iex> {b, c} {2, 3} iex> a %{"b" => 2, "c" => 3}

It supports map update syntax:

iex> person = %{"name" => "Jack", "age" => 26} iex> name = "John" iex> params(%{person | name, "age" => 28}) 

%{"name" => "John", "age" => 28}

iex> map = %{a: 1, b: 2, c: 3, d: 4} iex> {a, b} = {10, 20} iex> keys(%{map | a, b, c: 100, d: 200}) 

%{a: 10, b: 20, c: 100, d: 200} 

For more info you can find docs for keys and params on hexdocs.pm

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[Hassan Zamani]

One of main differences of Synex and ShortMaps is that Synex supports nested stractures:

iex> params(%{a => %{b, c}}) = %{"a" => %{"b" => 2, "c" => 3}} iex> {b, c} {2, 3} iex> a %{"b" => 2, "c" => 3}

It supports map update syntax:

iex> person = %{"name" => "Jack", "age" => 26} iex> name = "John" iex> params(%{person | name, "age" => 28}) 

%{"name" => "John", "age" => 28}

iex> map = %{a: 1, b: 2, c: 3, d: 4} iex> {a, b} = {10, 20} iex> keys(%{map | a, b, c: 100, d: 200}) 

%{a: 10, b: 20, c: 100, d: 200} 

For more info you can find docs for keys and params on hexdocs.pm

[Jaap Frolich]

Hi,

I like it! But I still think that there is a place for punning in the elixir language, as with this macro we are trading one simplification for an extra function call.

The main problem seems to be that there are two ways of pattern matching — by atom or by string. Perhaps it would be possible to first look for an atom key and if that key is not there look for a string key. There are not many use cases that you have identical keys both as a string and an atom in a map. We could then have: %{foo, bar, baz} = %{“foo” => “foo”, bar: “bar”, baz: “baz”} in the language.

Cheers,

Jaap

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