Changelog for Elixir v1.19

Type system improvements

Type checking of protocol dispatch and implementations

This release also adds type checking when dispatching and implementing protocols.

For example, string interpolation in Elixir uses the String.Chars protocol. If you pass a value that does not implement said protocol, Elixir will now emit a warning accordingly.

Here is an example passing a range, which cannot be converted into a string, to an interpolation:

defmodule Example do
  def my_code(first..last//step = range) do
    "hello #{range}"
  end
end

the above emits the following warnings:

warning: incompatible value given to string interpolation:

    data

it has type:

    %Range{first: term(), last: term(), step: term()}

but expected a type that implements the String.Chars protocol, it must be one of:

    dynamic(
      %Date{} or %DateTime{} or %NaiveDateTime{} or %Time{} or %URI{} or %Version{} or
        %Version.Requirement{}
    ) or atom() or binary() or float() or integer() or list(term())

Warnings are also emitted if you pass a data type that does not implement the Enumerable protocol as a generator to for-comprehensions:

defmodule Example do
  def my_code(%Date{} = date) do
    for(x <- date, do: x)
  end
end

will emit:

warning: incompatible value given to for-comprehension:

    x <- date

it has type:

    %Date{year: term(), month: term(), day: term(), calendar: term()}

but expected a type that implements the Enumerable protocol, it must be one of:

    dynamic(
      %Date.Range{} or %File.Stream{} or %GenEvent.Stream{} or %HashDict{} or %HashSet{} or
        %IO.Stream{} or %MapSet{} or %Range{} or %Stream{}
    ) or fun() or list(term()) or non_struct_map()

Type checking and inference of anonymous functions

Elixir v1.19 can now type infer and type check anonymous functions. Here is a trivial example:

defmodule Example do
  def run do
    fun = fn %{} -> :map end
    fun.("hello")
  end
end

The example above has an obvious typing violation, as the anonymous function expects a map but a string is given. With Elixir v1.19, the following warning is now printed:

    warning: incompatible types given on function application:

        fun.("hello")

    given types:

        binary()

    but function has type:

        (dynamic(map()) -> :map)

    typing violation found at:
    │
  6 │     fun.("hello")
    │        ~
    │
    └─ mod.exs:6:8: Example.run/0

Function captures, such as &String.to_integer/1, will also propagate the type as of Elixir v1.19, arising more opportunity for Elixir's type system to catch bugs in our programs.

Acknowledgements

The type system was made possible thanks to a partnership between CNRS and Remote. The development work is currently sponsored by Fresha, Starfish*, and Dashbit.

Faster compile times in large projects

This release includes two compiler improvements that can lead up to 4x faster builds in large codebases.

While Elixir has always compiled the given files in project or a dependency in parallel, the compiler would sometimes be unable to use all of the machine resources efficiently. This release addresses two common limitations, delivering performance improvements that scale with codebase size and available CPU cores.

Code loading bottlenecks

Prior to this release, Elixir would load modules as soon as they were defined. However, because the Erlang part of code loading happens within a single process (the code server), this would make it a bottleneck, reducing parallelization, especially on large projects.

This release makes it so modules are loaded lazily. This reduces the pressure on the code server and the amount of work during compilation, with reports of more than two times faster compilation for large projects. The benefits depend on the codebase size and the number of CPU cores available.

Implementation wise, the parallel compiler already acts as a mechanism to resolve modules during compilation, so we built on that. By making sure the compiler controls both module compilation and module loading, it can also better guarantee deterministic builds.

There are two potential regressions with this approach. The first one happens if you spawn processes during compilation which invoke other modules defined within the same project. For example:

defmodule MyLib.SomeModule do
  list = [...]

  Task.async_stream(list, fn item ->
    MyLib.SomeOtherModule.do_something(item)
  end)
end

Because the spawned process is not visible to the compiler, it won't be able to load MyLib.SomeOtherModule. You have two options, either use Kernel.ParallelCompiler.pmap/2 or explicitly call Code.ensure_compiled!(MyLib.SomeOtherModule) before spawning the process that uses said module.

The second one is related to @on_load callbacks (typically used for NIFs) that invoke other modules defined within the same project. For example:

defmodule MyLib.SomeModule do
  @on_load :init

  def init do
    MyLib.AnotherModule.do_something()
  end

  def something_else do
    ...
  end
end

MyLib.SomeModule.something_else()

The reason this fails is because @on_load callbacks are invoked within the code server and therefore they have limited ability to load additional modules. It is generally advisable to limit invocation of external modules during @on_load callbacks but, in case it is strictly necessary, you can set @compile {:autoload, true} in the invoked module to address this issue in a forward and backwards compatible manner.

Both snippets above could actually lead to non-deterministic compilation failures in the past, and as a result of these changes, compiling these cases are now deterministic.

Parallel compilation of dependencies

This release introduces a variable called MIX_OS_DEPS_COMPILE_PARTITION_COUNT, which instructs mix deps.compile to compile dependencies in parallel.

While fetching dependencies and compiling individual Elixir dependencies already happened in parallel, as outlined in the previous section, there were pathological cases where performance gains would be left on the table, such as when compiling dependencies with native code or dependencies where one or two large files would take most of the compilation time.

By setting MIX_OS_DEPS_COMPILE_PARTITION_COUNT to a number greater than 1, Mix will now compile multiple dependencies at the same time, using separate OS processes. Empirical testing shows that setting it to half of the number of cores on your machine is enough to maximize resource usage. The exact speed up will depend on the number of dependencies and the number of machine cores, although some reports mention up to 4x faster compilation times. If you plan to enable it on CI or build servers, keep in mind it will most likely have a direct impact on memory usage too.

Improved pretty printing algorithm

Elixir v1.19 ships with a new pretty printing implementation that tracks limits as a whole, instead of per depth. Previous versions would track limits per depth. For example, if you had a list of lists of 4 elements and a limit of 5, it would be pretty printed as follows:

[
  [1, 2, 3],
  [1, 2, ...],
  [1, ...],
  [...],
  ...
]

This allows for more information to be shown at different nesting levels, which is useful for complex data structures. But it led to some pathological cases where the limit option had little effect on filtering the amount of data shown. The new implementation decouples the limit handling from depth, decreasing it as it goes. Therefore, the list above with the same limit in Elixir v1.19 is now printed as:

[
  [1, 2, 3],
  ...
]

The outer list is the first element, the first nested list is the second, followed by three numbers, reaching the limit. This gives developers more precise control over pretty printing.

Given this may reduce the amount of data printed by default, the default limit has also been increased from 50 to 100. We may further increase it in upcoming releases based on community feedback.

Erlang/OTP 28 support

Elixir v1.19 officially supports Erlang/OTP 28.1+ and later. In order to support the new Erlang/OTP 28 representation for regular expressions, structs can now control how they are escaped into abstract syntax trees by defining a __escape__/1 callback.

On the other hand, the new representation for regular expressions implies they can no longer be used as default values for struct fields. Instead of this:

defmodule Foo do
  defstruct regex: ~r/foo/
end

You must do this:

defmodule Foo do
  defstruct [:regex]

  def new do
    %Foo{regex: ~r/foo/}
  end
end

OpenChain certification

Elixir v1.19 is also our first release following OpenChain compliance, as previously announced. In a nutshell:

Elixir releases now include a Source SBoM in CycloneDX 1.6 or later and SPDX 2.3 or later formats.
Each release is attested along with the Source SBoM.

These additions offer greater transparency into the components and licenses of each release, supporting more rigorous supply chain requirements.

This work was performed by Jonatan Männchen and sponsored by the Erlang Ecosystem Foundation.

v1.19.0 (2025-10-16)

1. Enhancements

Elixir

[Access] Add Access.values/0 for traversing maps and keyword lists values
[Base] Add functions to verify if an encoding is valid, such as valid16?, valid64?, and so forth
[Calendar] Support 2-arity options for Calendar.strftime/3 which receives the whole data type
[Code] Add :migrate_call_parens_on_pipe formatter option
[Code] Add :indentation option to Code.string_to_quoted/2
[Code.Fragment] Preserve more block content around cursor in container_cursor_to_quoted
[Code.Fragment] Add :block_keyword_or_binary_operator to Code.Fragment for more precise suggestions after operators and closing terminators
[Code.Fragment] Add Code.Fragment.lines/1
[Enum] Provide more information on Enum.OutOfBoundsError
[Inspect] Allow optional: :all when deriving Inspect
[Inspect.Algebra] Add optimistic/pessimistic groups as a simplified implementation of next_break_fits
[IO.ANSI] Add ANSI codes to turn off conceal and crossed_out
[Kernel] Raise when U+2028 and U+2029 characters are present in comments and strings to avoid line spoofing attacks
[Kernel] Include the line for the previous clause in errors/warnings related to conflicts between defaults on function definitions
[Kernel] Support min/2 and max/2 as guards
[Kernel.ParallelCompiler] Add each_long_verification_threshold which invokes a callback when type checking a module takes too long
[Kernel.ParallelCompiler] Include lines in == Compilation error in file ... == slogans
[Macro] Print debugging results from Macro.dbg/3 as they happen, instead of once at the end
[Macro] Add __escape__/1 callback so structs can escape references and other runtime data types in Macro.escape/1
[Module] Do not automatically load modules after their compilation, guaranteeing a more consistent compile time experience and drastically improving compilation times
[OptionParser] Support the :regex type
[OptionParser] Enhance parsing error to display available options
[Protocol] Type checking of protocols dispatch and implementations
[Regex] Add Regex.to_embed/2 which returns an embeddable representation of regex in another regex
[Regex] Raise error message when regexes are used as default values in struct fields for compatibility with Erlang/OTP 28
[Registry] Add key-based partitioning of duplicate registries
[String] Add String.count/2 to count occurrences of a pattern
[String] Update to Unicode 17.0.0

ExUnit

[ExUnit] Set a process label for each test
[ExUnit.CaptureLog] Parallelize log dispatch when multiple processes are capturing log
[ExUnit.Case] Add :test_group to the test context
[ExUnit.Doctest] Support ellipsis in doctest exceptions to match the remaining of the exception
[ExUnit.Doctest] Add :inspect_opts option for doctest

IEx

[IEx] Support multi-line prompts (due to this feature, :continuation_prompt and :alive_continuation_prompt are no longer supported as IEx configuration)
[IEx.Autocomplete] Functions annotated with @doc group: "Name" metadata will appear within their own groups in autocompletion

Logger

[Logger] Accept any enumerable in Logger.metadata/1

Mix

[mix] Add support for MIX_PROFILE_FLAGS to configure MIX_PROFILE
[mix compile] Debug the compiler and type checker PID when MIX_DEBUG=1 and compilation/verification thresholds are met
[mix compile] Add Mix.Tasks.Compiler.reenable/1
[mix deps.compile] Support MIX_OS_DEPS_COMPILE_PARTITION_COUNT for compiling deps concurrently across multiple operating system processes
[mix help] Add mix help Mod, mix help :mod, mix help Mod.fun, mix help Mod.fun/arity, and mix help app:package
[mix format] Add options to mix format to allow excluding of files
[mix test] Add --name-pattern option to mix test
[mix test] Allow to distinguish the exit status between warnings as errors and test failures
[mix xref graph] Add support for --format json
[mix xref graph] Emit a warning if --source is part of a cycle
[Mix] Support the :compilers option
[Mix.Task.Compiler] Add Mix.Task.Compiler.run/2

2. Bug fixes

Elixir

[Code] Return error on invalid unicode sequences in Code.string_to_quoted/2 instead of raising
[Code] Properly handle column annotation for in in not in expressions
[DateTime] Do not truncate microseconds regardless of precision in DateTime.diff/3
[Enum] Fix infinite loop on Enum.take/2 with negative index on empty enumerable
[File] Properly handle permissions errors cascading from parent in File.mkdir_p/1
[Inspect] Inspect ill-formed structs as maps
[Kernel] Properly increment metadata newline when ? is followed by a literal newline character
[Kernel] not_a_map.key now raises BadMapError for consistency with other map operations
[Protocol] defstruct/1 and defexception/1 are now disabled inside defprotocol as to not allow defining structs/exceptions alongside a protocol
[Regex] Fix Regex.split/2 returning too many results when the chunk being split on was empty (which can happen when using features such as /K)
[Stream] Ensure Stream.transform/5 respects suspend command when its inner stream halts
[URI] Several fixes to URI.merge/2 related to trailing slashes, trailing dots, and hostless base URIs

ExUnit

[ExUnit.Assertions] Fix order of pinned variables in failure reports
[ExUnit.Assertions] Raise if attempting to raise an assertion error with invalid message (not a binary)
[ExUnit.Case] Do not crash on empty test unit groups

IEx

[IEx] Abort pipelines when there is an error in any step along the way

Mix

[mix cmd] Preserve argument quoting in subcommands by no longer performing shell expansion. To revert to the previous behaviour, pass --shell before the command name
[mix compile] Fix bug where reverting changes to an external resource (such as HEEx template) after a compilation error would make it so the source module would not be compiled
[mix compile] Avoid failures when locking compilation across different users
[mix compile] Fix race condition when renaming files used by the compilation lock
[mix format] Ensure the formatter does not go over the specified limit in certain corner cases
[mix release] Fix RELEASE_SYS_CONFIG for Windows 11
[mix test] Ensure modules are preloaded in mix test --slowest-modules=N
[mix xref graph] Provide more consistent output by considering strong connected components only when computing graphs

3. Soft deprecations (no warnings emitted)

Elixir

[Inspect.Algebra] next_break_fits is deprecated in favor of optimistic/pessimistic groups
[Node] Node.start/2-3 is deprecated in favor of Node.start/2 with a keyword list

Mix

[mix compile] --no-protocol-consolidation is deprecated in favor of --no-consolidate-protocols for consistency with mix.exs configuration
[mix compile.protocols] Protocol consolidation is now part of compile.elixir and the task itself has no effect

4. Hard deprecations

Elixir

[Code] Warn if line-break characters outside of \r and \r\n are found in strings according to UX#55. This warning will be fast-tracked into an error for security reasons in Elixir v1.20, following a similar rule to bidirectional control characters. They will already raise if found in comments
[Code] The on_undefined_variable: :warn is deprecated. Relying on undefined variables becoming function calls will not be supported in the future
[File] Passing a callback as third argument to File.cp/3 is deprecated, pass it as a on_conflict: callback option instead
[File] Passing a callback as third argument to File.cp_r/3 is deprecated, pass it as a on_conflict: callback option instead
[Kernel] The struct update syntax, such as %URI{uri | path: "/foo/bar"}, now requires the given variable (or expression) to explicitly pattern match on the struct before it can be updated. This is because, thanks to the type system, pattern matching on structs can find more errors, more reliably, and we want to promote its usage. Once pattern matching is added, you may optionally convert the struct update syntax into the map update syntax %{uri | path: "/foo/bar"} with no less of typing guarantees
[Kernel.ParallelCompiler] Passing return_diagnostics: true as an option is required on compile, compile_to_path and require

Logger

[Logger] The :backends configuration is deprecated, either set the :default_handler to false or start backends in your application start callback

Mix

[mix] The :default_task, :preferred_cli_env, and :preferred_cli_target configuration inside def project in your mix.exs has been deprecated in favor of :default_task, :preferred_envs and :preferred_targets inside the def cli function
[mix do] Using commas as task separator in mix do (such as mix do foo, bar) is deprecated, use + instead (as in mix do foo + bar)

v1.18

The CHANGELOG for v1.18 releases can be found in the v1.18 branch.

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