Elixir

04Installing Elixir with the right Erlang/OTP

Elixir 1.20.4 needs Erlang/OTP 27 or later, and the 1.20 line is supported on OTP 27 to 29. The quickest route is the install script from elixir-lang.org, which sets up a matching Erlang as well. For projects pinned to different versions, the install page lists asdf and mise as version managers.

1. Install script for macOS, Ubuntu or Windows with bash. It installs Erlang and Elixir side by side, and you put both on your PATH:

   curl -fsSO https://elixir-lang.org/install.sh
   sh install.sh [email protected] [email protected]
   installs_dir=$HOME/.elixir-install/installs
   export PATH=$installs_dir/otp/28.4/bin:$PATH
   export PATH=$installs_dir/elixir/1.20.4-otp-28/bin:$PATH

2. With mise, pin Erlang and an Elixir build compiled for the same OTP major, which is the -otp-29 suffix:

   mise use erlang@29 [email protected]
   mise exec -- elixir --version

3. On Windows the official installer comes in one build per OTP major (27, 28, 29), so run erl first to see which Erlang you have. Scoop and Chocolatey work too:

   scoop install erlang
   scoop install elixir
   # or
   choco install elixir

4. In GitHub Actions, erlef/setup-beam reads the same pins from .tool-versions or mise.toml. With a version file it insists on version-type: strict:

   - uses: actions/checkout@v4
   - uses: erlef/setup-beam@v1
     with:
       version-file: .tool-versions
       version-type: strict

The table is a support promise per minor: 1.19 is supported on OTP 26 to 28. Moving a machine to OTP 29 means moving to Elixir 1.20 with it.

Install Elixir / Compatibility and deprecations / mise: Elixir / erlef/setup-beam

05Which Elixir versions still get security fixes

Bug fixes go to the newest minor only, which is 1.20. Security patches are promised for the last five minors, 1.16 to 1.20. For CVE-2026-75758 the record is shorter: the advisory lists every release from 1.15.0 up to the fixes as affected, and fixed releases exist for 1.18, 1.19 and 1.20 only.

1. Check which Elixir and Erlang/OTP you run:

   elixir --version

2. Move to the fixed release on your branch: 1.18.5, 1.19.6 or 1.20.4. On 1.16 or 1.17 there is no fixed release, so the fix is an upgrade to 1.18.5 or later.
3. New releases go to the read-only announcements mailing list, and security releases are tagged [security] there.

The bug: inspect/1 checks only the first 4,096 elements of a list before treating it as text. A longer printable list with one invalid element at the end fails the conversion, and the error path recurses until the process hits its heap limit or the node runs out of memory. The advisory rates it moderate.

GHSA-jf5q-v438-665c / Compatibility and deprecations / 1.20.4 notes / 1.18.5 notes

06What the type checker catches, 1.17 to 1.20

Since 1.17 the compiler runs a set-theoretic type checker over code that has no type annotations. Each minor taught it more. 1.20 infers types for every construct and reports what the announcement calls verified bugs: typing violations that are guaranteed to fail at runtime.

1. A verified bug, from the 1.20 announcement. data.a narrows data to a map with an a field, so using data itself as a number is a violation and the compiler warns:

   def add_a_and_b(data) do
     data.a + data
   end

2. New releases add new warnings, and the compatibility page warns that these fail builds that run with --warnings-as-errors. Upgrade the compiler on a branch first:

   mix compile --warnings-as-errors

3. 1.20 hard-deprecated five things. One of them: a size from a variable in a binary pattern now needs the pin operator, which works since 1.15:

   <<x::size(^y)>> = bin

Type signatures have no date. The 1.20 announcement sets four conditions first: good checker performance, plus efficient recursive types, parametric types and traversal of map key-value pairs.

1.17 announcement / 1.18 announcement / 1.19 announcement / 1.20 announcement / Deprecations table

07Faster compiles on 1.19 and 1.20

Two settings are worth trying. On 1.19 and later, MIX_OS_DEPS_COMPILE_PARTITION_COUNT compiles dependencies in parallel OS processes, and the 1.19 notes say half your cores is enough. On 1.20, module_definition: :interpreted runs module bodies in the interpreter. The notes say it may cut compile time in large projects and leaves the .beam files unchanged.

1. Compile dependencies in parallel. Expect more memory use, which matters on CI runners:

   MIX_OS_DEPS_COMPILE_PARTITION_COUNT=4 mix deps.compile

2. Interpret module bodies, in def project of mix.exs:

   elixirc_options: [module_definition: :interpreted]

3. If a build broke on 1.19: modules now load lazily. Code that spawns processes at compile time and calls other modules of the same project needs Kernel.ParallelCompiler.pmap/2 or Code.ensure_compiled!/1. An @on_load callback that calls project modules needs @compile {:autoload, true} in the module it calls.

The 1.19 announcement cites reports of more than 2x faster builds from lazy loading and up to 4x with parallel dependencies, both on large projects.

1.19 announcement / 1.20 announcement

08How to exit IEx

Press Ctrl+C twice. The IEx docs list two more ways: Ctrl+C opens the BREAK menu, where q and Enter quits, and Ctrl+\ quits as well. A remote shell keeps running after you disconnect.

IEx forgets its history between sessions unless the VM is told to keep it:

iex --erl "-kernel shell_history enabled", or set ERL_AFLAGS="-kernel shell_history enabled" in your shell profile.

IEx docs: exiting the shell / IEx docs: shell history