Aetherus

Aetherus

I want to mimic ES6’s destructing assignment using Elixir’s sigil.

What I want to achieve is

~m{foo bar} = %{foo: 1, bar: 2}
foo  #=> 1
bar  #=> 2

So far I have the following code

defmodule DestructingAssignment do
  defmacro __using__(_) do
    quote do: import unquote(__MODULE__)
  end

  defmacro sigil_m({:<<>>, _line, [string]}, []) do
    spec = string
           |> String.split
           |> Stream.map(&String.to_atom/1)
           |> Enum.map(&{&1, {&1, [], Elixir}})  #=> [foo: {:foo, [], Elixir}, bar: {:bar, [], Elixir}]
    {:%{}, [], spec}
  end
end

When I run ~m{foo bar} = %{foo: 1, bar: 2}, pattern matching succeeds. But when I try to read the variable foo, it gives me

warning: variable "foo" does not exist and is being expanded to "foo()", please use parentheses to remove the ambiguity or change the variable name
  iex:9

** (CompileError) iex:9: undefined function foo/0

I guess this is due to the macro hygiene, but as you can see, the sigil implementation doesn’t use quote. How can I inject local variable to the macro’s caller’s scope?

Showing Posts 1 to 6

kip

kip

ex_cldr Core Team

I think just one small change is required:

defmodule DestructingAssignment do
  defmacro __using__(_) do
    quote do: import unquote(__MODULE__)
  end

  defmacro sigil_m({:<<>>, _line, [string]}, []) do
    spec = string
           |> String.split
           |> Stream.map(&String.to_atom/1)
           |> Enum.map(&{&1, {&1, [], nil}})  #=> [foo: {:foo, [], nil}, bar: {:bar, [], nil}]
    {:%{}, [], spec}
  end
end
Aetherus

Aetherus OP

Works like a charm! But here come a new question, what does the last part of an AST node mean?

taiansu

taiansu

According to Saša Jurić’s article about macros, the third part of variable AST represents where the quoting happens. If it’s nil, the identifier is not hygienic. Check out paragraph “Discovering the AST structure” of the article and the series for the detail.

KristerV

KristerV

I have no ideas how macros work, but this is very useful and I made a string version also:

  defmacro sigil_d({:<<>>, _line, [string]}, []) do
    spec =
      string
      |> String.split()
      |> Stream.map(&String.to_atom/1) # how could we do without this?
      |> Enum.map(&{to_string(&1), {&1, [], nil}})

    {:%{}, [], spec}
  end

Works great! (so far)

iex(31)> ~d(a) = %{"a" => 5}
%{"a" => 5}
iex(32)> a
5
KristerV

KristerV

can’t edit.

this algo has served me well, but today i’d go for shorter_maps.

— All posts loaded —

Where Next? Top

Trending in Questions Top

water
I’m aware of 2 such projects, are there others? git: GitHub - team-alembic/realworld: A fullstack Phoenix LiveView application with back...
New
katta
I having some trouble figuring out if I have set myself too strict of standards for my production server. Currently I can handle 75% of r...
New
peck
Hi everyone. My team and I have been working on a fairly modest app based around video streaming and chat, but we’ve landed a customer t...
New
bradley
I really like the adapter patterns that ecto, nebulex, waffle, etc. use and would love find something similar for a key management servic...
New
unaware8150
Hello folks! So at work, we are seeing some situations where we have to define some “fixed” strings that are used across the codebase in...
New
Alvinkariuki
How Can I Optimise Compile Time Dependencies I have been building an elixir application for about 2 years now. Many modules and files ha...
New

Other Trending Topics Top

budgie
A little off-topic, but I feel like people here have a good head on their shoulders. I used to be quite good at making software. Was luc...
New
KristerV
Hey. Is there anyone here who creates agents in their apps? Not talking about using agents, but creating them. I’m finding it pretty diff...
New
mudasobwa
I fully migrated to my own harness from Anthropic/Gemini and I think it’s time to share it. Welcome DSH, the DeepSeek Harness, fully writ...
New
jimsynz
Beam Bots (or just BB for short) is a framework for building fault-tolerant robotics applications in Elixir using familiar OTP patterns. ...
New
juhalehtonen
There has been a thread to discuss the Stack Overflow Developer Survey on this forum every year since 2018, so here’s yet another one for...
New
budgie
I love Elixir. It’s one of 2 programming languages I’ve ever fallen in love with. But I don’t use it anymore. Serverless was the promis...
New

We're in Beta

About us Mission Statement

Options

Thread Display Mode




Thread Preview

Skip Thread Previews