es6_maps

Enables ES6-like shorthand usage of Elixir maps.

Why?

When writing code that heavily utilizes structures and passes complex objects through multiple layers, it's common to frequently use map literals. This often results in repetitive code patterns such as ctx = %{variable: variable, user: user, ...} or %{variable: variable, user: user, ...} = ctx.

I believe that introducing a shorthand form of object creation to Elixir enhances the language's ergonomics and is a natural extension of its existing map literals syntax. This feature will be immediately familiar to JavaScript and Rust developers, and similar shorthands are present in other languages such as Go.

Is there any runtime overhead?

No; the shorthand map keys compile down to exactly the same bytecode as the "vanilla-style" maps.

Installation

The package can be installed by adding es6_maps to your list of dependencies and compilers in mix.exs:

# mix.exs

def project do
  [
    compilers: [:es6_maps | Mix.compilers()],
    deps: deps()
  ]
end

def deps do
  [
    {:es6_maps, "~> 0.2.2", runtime: false}
  ]
end

Usage

Creating maps

iex> {hello, foo, bar} = {"world", 1, 2}
iex> %{hello, foo, bar: bar}
%{hello: "world", foo: 1, bar: 2}

Destructuring maps

iex> %{hello, foo} = %{hello: "world", foo: 1, bar: 2}
iex> hello
"world"
iex> foo
1

Updating maps

iex> map = %{hello: "world", foo: 1, bar: 2}
iex> foo = :baz
iex> %{map | foo, bar: :bong}
%{hello: "world", foo: :baz, bar: :bong}

Structs

All of the above work for structs as well:

defmodule MyStruct do
  defstruct [:hello, :foo, :bar]
end

iex> {foo, bar} = {1, 2}
iex> %MyStruct{foo, bar, hello: "world"}
%MyStruct{foo: 1, bar: 2, hello: "world"}

iex> struct = %MyStruct{foo: 1, bar: 2}
iex> hello = "world"
iex> %MyStruct{struct | hello}
%MyStruct{foo: 1, bar: 2, hello: "world"}

iex> %MyStruct{hello} = %MyStruct{hello: "world", foo: 1}
iex> hello
"world"

Converting existing code to use ES6-style maps

es6_maps includes a formatting plugin that will convert your existing map and struct literals into the shorthand style. Add the plugin to .formatter.exs, then call mix format to reformat your code:

# .formatter.exs
[
  plugins: [Es6Maps.Formatter],
  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
]

The plugin manipulates the AST, not raw strings, so it's precise and will only change your code by:

1. changing map keys into the shorthand form;
2. reordering map keys so the shorthand form comes first;
3. formatting the results like mix format would.

Reverting to the vanilla-style maps

The formatting plugin can also be used to revert all of the ES6-style map shorthand uses back to the "vanilla" style. Set the es6_maps: [map_style: :vanilla] option in .formatter.exs, then call mix format to reformat your code:

# .formatter.exs
[
  plugins: [Es6Maps.Formatter],
  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
  es6_maps: [map_style: :vanilla]
]

Formatting pragmas

The plugin supports pragmas in the comments to control the formatting. The pragma must be in the form # es6_maps: [map_style: :es6] and can be placed anywhere in the file. The map_style option can be set to :es6 to convert to shorthand form or :vanilla to revert to the vanilla-style maps. The pragma takes effect only on the line following the comment.

For example in the code below, the first map will be formatted to the shorthand form, while the second map will be left as is:

  %{foo, bar: 1} = var
  # es6_maps: [map_style: :vanilla]
  %{hello: hello, foo: foo, bar: 1} = var

es6_maps: [map_style: :vanilla] option in .formatter.exs can be combined with # es6_maps: [map_style: :es6] comment pragmas.

How does it work?

es6_maps replaces in runtime the Elixir compiler's elixir_map module. The module's expand_map/4 function is wrapped with a function that replaces map keys %{k} as if they were %{k: k}. After es6_maps runs as one of the Mix compilers, the Elixir compiler will use the replaced functions to compile the rest of the code.

[!IMPORTANT] By the nature of this solution it's tightly coupled to the internal Elixir implementation. The current version of es6_maps should work for Elixir 1.15, 1.16 and the upcoming 1.17 version, but may break in the future.