sfrances
Using module attributes in typespec definitions to reduce duplication?
Hi
I’m trying to use module attributes in my typespec, but I get errors when I do this:
defmodule KitchenCalculator do
@ml :milliliter
@cup :cup
@fluid_ounce :fluid_ounce
@tsp :teaspoon
@tbsp :tablespoon
@type unit :: @ml | @cup | @fluid_ounce | @tsp | @tbsp
# More stuff here
end
I get the error:
** (CompileError) lib/kitchen_calculator.ex:8: type ml/0 undefined (no such type in KitchenCalculator)
(elixir 1.12.2) lib/kernel/typespec.ex:925: Kernel.Typespec.compile_error/2
(stdlib 3.15.2) lists.erl:1358: :lists.mapfoldl/3
(elixir 1.12.2) lib/kernel/typespec.ex:834: Kernel.Typespec.typespec/4
(stdlib 3.15.2) lists.erl:1358: :lists.mapfoldl/3
(elixir 1.12.2) lib/kernel/typespec.ex:464: Kernel.Typespec.typespec/4
(elixir 1.12.2) lib/kernel/typespec.ex:307: Kernel.Typespec.translate_type/2
(stdlib 3.15.2) lists.erl:1358: :lists.mapfoldl/3
(elixir 1.12.2) lib/kernel/typespec.ex:235: Kernel.Typespec.translate_typespecs_for_module/2
I use the module attributes later on, to reduce duplication and chance of errors via typos when typing out atoms. Is there any way to achieve this, or do I just live with the minor duplication?
Marked As Solved
kip
You can do it like this:
defmodule KitchenCalculator do
@ml :milliliter
@cup :cup
@fluid_ounce :fluid_ounce
@tsp :teaspoon
@tbsp :tablespoon
types = Enum.reduce([@ml, @cup, @fluid_ounce, @tsp, @tbsp], &({:|, [], [&1, &2]}))
@type unit :: unquote(types)
# More stuff here
end
You could of course encapsulate the Enum.reduce/2 into a macro and reuse it that way.
iex> t KitchenCalculator.unit
@type unit() :: :tablespoon | :teaspoon | :fluid_ounce | :cup | :milliliter
BTW, if you’re spending a lot of time with units, unit math and unit conversions, and potentially unit serialisation and localization, you might find ex_cldr_units helpful (I’m the author).
Also Liked
kip
In the few occasions when I’ve used the technique above I have done it directly. In my cases, building a macro just to do that hasn’t been important and I felt it would actually make the code more obscure. By having the Metaprogramming line directly above, it makes it easy for “future me” to work out what I was thinking.
I’m sure you’ve heard the aphorism in Elixir-land, “first rule of macros is don’t use macros”.
Popular in Questions
Other popular topics
Latest on Elixir Forum
Sponsor Spotlight
Supporting innovation across the BEAM ecosystem.
Categories:
Sub Categories:
Forums
Popular Tags
- #ecto
- #liveview
- #troubleshooting
- #learning-elixir
- #deployment
- #library
- #erlang
- #testing
- #genserver
- #mix
- #absinthe
- #remote-other
- #otp
- #plug
- #how-to-question
- #macros
- #postgres
- #channels
- #elixirconf
- #exunit
- #discussion
- #code-sync
- #javascript
- #podcasts
- #onsite
- #dialyzer
- #docker
- #authentication
- #umbrella
- #full-time-contract
- #podcasts-by-brainlid
- #ecto-query
- #elixir-ls
- #phoenix_html
- #iex
- #blog-post
- #graphql
- #genstage
- #ai
- #websockets
- #supervisor
- #advent-of-code
- #elixirconf-us
- #distillery
- #processes
- #forms
- #api
- #metaprogramming
- #security
- #performance
Our Sponsors
Build Elixir applications with speed and confidence.
Supporting innovation across the BEAM ecosystem.
We build reliable cloud platforms for business-critical systems.
Catch errors, track performance, monitor hosts and more.
Real-time error tracking, performance insights, and observability for devs.
Practical resources that improve the lives of professional developers.
Develop your skills with books, videos, and courses.
Enabling companies to succeed by building software people love.
Courses that'll move you from confusion to "Aha, now I get it!"