iamacube
Understanding improper lists usage in elixir-plug
I want to understand the reason of constructing an improper list usage in the elixir-plug repository, Line 256
First Question: Is the usage of an improper list necessary here?:
# encoder fallback
defp encode_pair(field, value, encoder) do
[field, ?= | encode_value(value, encoder)]
end
I think improper lists do not have many uses; also there are many functions that will error when given an improper list.
for example Dialyzer would raise a warning for constructing an improper list, and they got this -dialyzer({no_improper_lists, g/0}). to suppress the warning.
Second Question: why this construct actually exists in Erlang?
Most Liked
benwilson512
You might not be wrong about this function, it just isn’t super obvious to me that Dialyzer is right either. Either way, check out this line: plug/lib/plug/conn/query.ex at 274e44f9a149b922099bf60029d8267afe494968 · elixir-plug/plug · GitHub
The encode_pair function is a defp and the output of that function is passed immediately to IO.iodata_to_binary. iodata is one of the real world uses for improper lists, because it reduces the overhead of building out a list front to back. The iodata_to_binary is able to handle improper lists and turn them into a single binary.
Dialyzer warns about improper lists because most of the time you do indeed want to warn for them. If the Plug core team used dialyzer they would probably put a no-warn invocation in there to tell dialyzer that this is intentional.
benwilson512
I don’t think this does create an improper list. It’s a recursive function, and that function has termination clauses that return [], which means it’ll construct a proper list.
As to this:
I think it’s better to approach this by understanding that the cons operator | is, at least in erlang, more foundational than even lists. Its most common use is with lists, but it has uses for low memory btrees, and gets use in IOlists as well.
sabiwara
Sorry for resurrecting an old thread, but while looking into this I also stumbled upon this issue which is relevant I think:
https://github.com/erlang/otp/issues/5937
The OTP team seems to be considering disabling these warnings, since there are legit use cases for improper lists:
- IO data as already explained above
- as a low-level performance optimization for libraries:
[a | b]takes less memory than{a, b}
Popular in Questions
Other popular topics
Latest on Elixir Forum
Sponsor Spotlight
Build Elixir applications with speed and confidence.
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
- #elixirconf-us
- #advent-of-code
- #distillery
- #processes
- #forms
- #api
- #metaprogramming
- #hex
- #security
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.
Error tracking for Elixir devs who love to ship. Start your free account.
Practical resources that improve the lives of professional developers.
Enabling companies to succeed by building software people love.
The deployment platform built for Elixir: PaaS ease, VPS control.
The team behind Membrane, Popcorn, LiveDebugger. Available for hire.
Courses that'll move you from confusion to "Aha, now I get it!"