Changelog for Elixir v1.18

Elixir v1.18 is an impressive release with improvements across the two main efforts happening within the Elixir ecosystem right now: set-theoretic types and language servers. It also comes with built-in JSON support and adds new capabilities to its unit testing library. Here is a quick break down.

Type system improvements

The most exciting change in Elixir v1.18 is type checking of function calls, alongside gradual inference of patterns and return types. To understand how this will impact your programs, consider the following code in "lib/user.ex":

defmodule User do
  defstruct [:age, :car_choice]

  def drive(%User{age: age, car_choice: car}, car_choices) when age >= 18 do
    if car in car_choices do
      {:ok, car}
    else
      {:error, :no_choice}
    end
  end

  def drive(%User{}, _car_choices) do
    {:error, :not_allowed}
  end
end

Elixir's type system will infer that the drive/2 function expects a %User{} struct and returns either {:ok, dynamic()}, {:error, :no_choice}, or {:error, :not_allowed}.

Therefore, the following code in a separate module (either in a separate or the same file), should emit a violation, due to an invalid argument:

User.drive({:ok, %User{}}, car_choices)

Here is the warning:

    warning: incompatible types given to User.drive/2:

        User.drive({:ok, %User{age: nil, car_choice: nil}}, car_choices)

    given types:

        {:ok, %User{age: nil, car_choice: nil}}, empty_list()

    but expected one of:

        dynamic(%User{age: term(), car_choice: term()}), dynamic()

    where "car_choices" was given the type:

        # type: empty_list()
        # from: lib/foo.ex:21:17
        car_choices = []

    typing violation found at:
    │
 22 │     User.drive({:ok, %User{}}, car_choices)
    │          ~
    │
    └─ lib/foo.ex:22:10: Example.run/0

The mismatched arguments are shown in red, if your terminal supports ANSI coloring.

And the next snippet will warn because the :error clause will never match, as that's not a valid return type of the User.drive/2 call:

case User.drive(user, car_choices) do
  {:ok, car} -> car
  :error -> Logger.error("User cannot drive")
end

And here is the warning:

    warning: the following clause will never match:

        :error

    because it attempts to match on the result of:

        User.drive(user, car_choices)

    which has type:

        dynamic({:ok, term()} or {:error, :no_choice} or {:error, :not_allowed})

    typing violation found at:
    │
 26 │       :error -> Logger.error("User cannot drive")
    │       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/foo.ex:26: Example.run/0

For more details on typing inference and the trade-offs made by the Elixir team, see our official documentation.

There are many other improvements to the type system, which we will go in detail within the official release. Meanwhile, here is a list summary of the overall improvements done to the type system:

Type inference of patterns (typing inference of guards will be part of an upcoming release)
Type checking of all language constructs, including local and remote calls, except for, with, and closures
Type checking of all functions inlined by the compiler found in Kernel
Type checking of all conversion functions inlined by the compiler
Support for tuples and lists as composite types as well as type checking of their basic operations
Detection of clauses and patterns that will never match from case, cond, and =
Detection of unused clauses in private functions

ExUnit improvements

ExUnit now supports parameterized tests to run the same test module multiple times under different parameters.

For example, Elixir ships a local, decentralized and scalable key-value process storage called Registry. The registry can be partitioned and its implementation differs depending if partitioning is enabled or not. Therefore, during tests, we want to ensure both modes are exercised. With Elixir v1.18, we can achieve this by writing:

defmodule Registry.Test do
  use ExUnit.Case,
    async: true,
    parameterize: [
      %{partitions: 1},
      %{partitions: 8}
    ]

  # ... the actual tests ...
end

ExUnit parameterizes whole test modules. If your modules are configured to run concurrently, as above, so will the parameterized ones.

ExUnit also comes with the ability of specifying test groups. While ExUnit supports running tests concurrently, those tests must not have shared state between them. However, in large applications, it may be common for some tests to depend on some shared state, and other tests to depend on a completely separate state. For example, part of your tests may depend on Cassandra, while others depend on Redis. Prior to Elixir v1.18, these tests could not run concurrently, but in v1.18 they might as long as they are assigned to different groups. Tests modules within the same group do not run concurrently, but across groups, they might.

With features like async tests, suite partitioning, and now grouping, Elixir developers have plenty of flexibility to make the most use of their machine resources, both in development and in CI.

`mix format --migrate`

The mix format command now supports an explicit --migrate flag, which will convert constructs that have been deprecated in Elixir to their latest version. Because this flag rewrites the AST, it is not guaranteed the migrated format will always be valid when used in combination with macros that also perform AST rewriting.

As of this release, the following migrations are executed:

Normalize parens in bitstring modifiers - it removes unnecessary parentheses in known bitstring modifiers, for example <<foo::binary()>> becomes <<foo::binary>>, or adds parentheses for custom modifiers, where <<foo::custom_type>> becomes <<foo::custom_type()>>.
Charlists as sigils - formats charlists as ~c sigils, for example 'foo' becomes ~c"foo".
unless as negated ifs - rewrites unless expressions using if with a negated condition, for example unless foo do becomes if !foo do.

More migrations may be added in future releases.

JSON support

This release includes official support for JSON encoding and decoding.

Both encoder and decoder fully conform to RFC 8259 and ECMA 404 standards.

Encoding

Encoding can be done via JSON.encode!/1 and JSON.encode_to_iodata!/1 functions. The default encoding rules are applied as follows:

Elixir	JSON
`integer() \| float()`	Number
`true \| false`	Boolean
`nil`	Null
`binary()`	String
`atom()`	String
`list()`	Array
`%{String.Chars.t() => _}`	Object

You may also implement the JSON.Encoder protocol for custom data structures. Elixir already implements the protocol for all Calendar types.

If you have a struct, you can derive the implementation of the JSON.Encoder by specifying which fields should be encoded to JSON:

  @derive {JSON.Encoder, only: [...]}
  defstruct ...

Decoding

Decoding can be done via JSON.decode/2 and JSON.decode!/2 functions. The default decoding rules are applied as follows:

JSON	Elixir
Number	`integer() \| float()`
Boolean	`true \| false`
Null	`nil`
String	`binary()`
Object	`%{binary() => _}`

Language server listeners

4 months ago, we welcomed the Official Language Server team, with the goal of unifying the efforts behind code intelligence, tools, and editors in Elixir. Elixir v1.18 brings new features on this front by introducing locks and listeners to its compilation. Let's understand what it means.

At the moment, all language server implementations have their own compilation environment. This means that your project and dependencies during development are compiled once, for your own use, and then again for the language server. This duplicate effort could cause the language server experience to lag, when it could be relying on the already compiled artifacts of your project.

This release address by introducing a compiler lock, ensuring that only a single operating system process running Elixir compiles your project at a given moment, and by providing the ability for one operating system process to listen to the compilation results of others. In other words, different Elixir instances can now communicate over the same compilation build, instead of racing each other.

These enhancements do not only improve editor tooling, but they also directly benefit projects like IEx and Phoenix. For example, you can invoke IEx.configure(auto_reload: true) and IEx will automatically reload modules changed elsewhere, either by a separate terminal or your IDE.

Potential incompatibilities

This release no longer supports WERL (a graphical user interface on Windows used by Erlang 25 and earlier). For a better user experience on Windows terminals, use Erlang/OTP 26+ (this is also the last Elixir release to support Erlang/OTP 25).

Furthermore, in order to support inference of patterns, Elixir will raise if it finds recursive variable definitions. This means patterns that never match, such as this one, will no longer compile:

def foo(x = {:ok, y}, x = y)

However, recursion of root variables (where variables directly point to each other), will also fail to compile:

def foo(x = y, y = z, z = x)

While the definition above could succeed (as long as all three arguments are equal), the cycle is not necessary and could be removed, as below:

def foo(x = y, y = z, z)

You may also prefer to write using guards:

def foo(x, y, z) when x == y and y == z

v1.18.3 (2025-03-06)

1. Enhancements

Elixir

[JSON] Encode any JSON key to string
[Kernel] Allow <<_::3*8>> in typespecs

Mix

[mix loadpaths] Support --no-listeners option

2. Bug fixes

Elixir

[CLI] Fix --no-color not setting :ansi_enabled to false
[Protocol] Return correct implementation for an invalid struct pointing to nil
[Stream] Do not raise when Stream.cycle/1 is explicitly halted

ExUnit

[ExUnit.Diff] Fix regression when diffing nested improper lists

IEx

[IEx.Autocomplete] Fix autocomplete crash when expanding struct with __MODULE__
[IEx.Helpers] Do not purge on recompile if IEx is not running

v1.18.2 (2025-01-22)

1. Enhancements

Elixir

[CLI] Add --color/--no-color for enabling and disabling of ANSI colors
[Code.Fragment] Provide more AST context when invoking container_cursor_to_quoted with trailing fragments
[Regex] Ensure compatibility with Erlang/OTP 28+ new Regex engine

Mix

[mix] Print compilation lock waiting message to stderr
[mix] Add an environment variable to optionally disable compilation locking

2. Bug fixes

Elixir

[CLI] Temporarily remove PowerShell scripts for elixir, elixirc, and mix on Windows, as they leave the shell broken after quitting Erlang

ExUnit

[ExUnit] Fix crash when diffing bitstring specifiers

IEx

[IEx.Autocomplete] Fix crashing when autocompleting structs with runtime values

Mix

[mix] Track compilation locks per user to avoid permission errors
[mix deps.update] Ensure Git dependencies can be upgraded by doing so against the origin

v1.18.1 (2024-12-24)

1. Enhancements

[Kernel] Do not emit type violation warnings when comparing or matching against literals
[Kernel] Do not validate clauses of private overridable functions

2. Bug fixes

Elixir

[Code.Fragment] Ensure Code.Fragment.container_cursor_to_quoted/2 with :trailing_fragment parses expressions that were supported in previous versions
[Kernel] Do not crash when typing violation is detected on dynamic dispatch
[Kernel] Properly annotate the source for warnings emitted by the compiler with the @file annotation
[Kernel] Properly annotate the source for warnings emitted by the type system with the @file annotation
[Kernel] Remove :no_parens metadata when using capture with arity on all cases
[Kernel] Ensure diagnostic traces are kept backwards compatible

ExUnit

[ExUnit.Case] Ensure async groups do not run concurrenly while the test suite is still loading
[ExUnit.Case] Ensure --repeat-until-failure can be combined with groups

Mix

[mix compile.elixir] Store compilation results if compilation fails due to --warnings-as-errors
[mix deps.loadpaths] Add build lock
[mix escript.build] Ensure build succeeds when protocol consolidation is disabled
[Mix.Shell] Ensure encoding is properly respected on Windows and Unix systems

v1.18.0 (2024-12-19)

1. Enhancements

Elixir

[CLI] Add experimental PowerShell scripts for elixir, elixirc, and mix on Windows. Those provide a safer entry point for running Elixir from other platforms
[Calendar] Add Duration.to_string/1
[Code] Support several migration options in Code.format_string!/2
[Code] Add parenthesis around -- and --- in Code.format_string!/2 to make precedence clearer
[Code] Include more metadata in Code.string_to_quoted/2 when token_metadata: true to help compute ranges from the AST
[Code.Fragment] Have :capture_arg as its own entry in Code.Fragment.surround_context/2
[Config] Add Config.read_config/1
[Enumerable] Add Enum.product_by/2 and Enum.sum_by/2
[Exception] Add MissingApplicationsError exception to denote missing applications
[JSON] Add a new JSON module with encoding and decoding functionality
[JSON] Implement JSON.Encoder for all Calendar types
[Kernel] Update source code parsing to match UTS #55 latest recommendations. In particular, mixed script is allowed in identifiers as long as they are separate by underscores (_), such as http_сервер. Previously allowed highly restrictive identifiers, which mixed Latin and other scripts, such as the japanese word for t-shirt, Tシャツ, now require the underscore as well
[Kernel] Warn on bidirectional confusability in identifiers
[Kernel] Verify the type of the binary generators
[Kernel] Track the type of tuples in patterns and inside elem/2
[Kernel] Perform validation of root AST nodes in unquote and unquote_splicing to catch bugs earlier
[Kernel] Add source, behaviour, and record information to Docs chunk metadata
[Kernel] Support deterministic builds in tandem with Erlang by setting ERL_COMPILER_OPTIONS=deterministic. Keep in mind deterministic builds strip source and other compile time information, which may be relevant for programs
[Kernel] Allow aliases and imports to be enabled conditionally in module body
[List] Add List.ends_with?/2
[Macro] Improve dbg handling of if/2, with/1 and of code blocks
[Macro] Add Macro.struct_info!/2 to return struct information mirroring mod.__info__(:struct)
[Registry] Add Registry.lock/3 for local locking
[PartitionSupervisor] Add PartitionSupervisor.resize!/2 to resize the number of partitions in a supervisor (up to the limit it was started with)
[Process] Handle arbitrarily high integer values in Process.sleep/1
[Protocol] Add @undefined_impl_description to customize error message when an implementation is undefined
[Protocol] Add __deriving__/1 as optional macro callback to Protocol, no longer requiring empty implementations
[String] Inspect special whitespace and zero-width characters using their Unicode representation
[String] Update Unicode to 16.0

ExUnit

[ExUnit] Support parameterized tests on ExUnit.Case
[ExUnit] Support test groups: tests in the same group never run concurrently
[ExUnit.Case] Add test_pid as a tag

IEx

[IEx] Add IEx.configure(auto_reload: true) to automatically pick up modules recompiled from other operating system processes
[IEx] Add :dot_iex support to IEx.configure/1
[IEx] Add report for normal/shutdown exits in IEx

Mix

[mix compile] Ensure only a single operating system process can compile at a given time
[mix deps.get] Ensure only a single operating system process can fetch deps at a given time
[mix format] Add mix format --migrate to migrate from deprecated functionality
[mix format] Add new options and metadata to improve formatting applying by editors and other environments
[mix test] Taint failure manifest if requiring or compiling tests fail
[Mix.Project] Add a :listeners configuration to listen to compilation events from the current and other operating system processes
[Mix.Task.Compiler] Add API for fetching all persisted compiler diagnostics
[Mix.Task.Compiler] Add API for fetching all compiler tasks

2. Bug fixes

Elixir

[Code] Fix delimiter metadata for single quoted atoms and remote calls in Code.string_to_quoted/2
[Code.Formatter] Fix formatter adding extra escapes to quoted remote calls
[Code.Fragment] Properly handle keyword keys as their own entry
[Inspect.Algebra] Ensure next_break_fits respects line_length
[Kernel] Validate AST on unquote and unquote_splicing to provide better error reports instead of failing too late inside the compiler
[Kernel] Avoid crashes when emitting diagnostics on code using \t for indentation
[Module] Include module attribute line and name when tracing its aliases
[Stream] Do not halt streams twice in Stream.transform/5
[URI] Fix a bug when a schemaless URI is given to URI.merge/2

ExUnit

[ExUnit.Assertions] Raise if guards are used in assert/1 with =
[ExUnit.Assertions] Format inserted/deleted maps in list assertions

IEx

[IEx.Helpers] IEx.Helpers.recompile/0 will reload modules changed by other operating system processes

Mix

[mix compile] Ensure warnings from external resources are emitted with --all-warnings when files do not change
[mix deps.compile] Fix escaping issues when invoking rebar3 in some cases
[mix escript] Fix escript layout and support storing priv directories
[mix release] Make .app files deterministic in releases
[Mix.Shell] Fix Mix.Shell on Windows when outputting non UTF-8 characters

3. Soft deprecations (no warnings emitted)

Elixir

[Inspect.Algebra] color/3 is deprecated in favor of color_doc/3
[Inspect.Algebra] fold_doc/2 is deprecated in favor of fold/2
[Kernel] Deprecate unless in favor of if. Use mix format --migrate to automate the migration
[Macro] Macro.struct!/2 is deprecated in favor of Macro.struct_info!/2
[Protocol] Defining __deriving__/3 inside the Any implementation is deprecated, derive it inside the protocol definition itself

4. Hard deprecations

EEx

[EEx] <%# is deprecated in favor of <%!-- or <% #
[EEx] c:EEx.handle_text/2 is deprecated in favor of c:EEx.handle_text/3

Elixir

[Code] Setting :warnings_as_errors is deprecated via Code.put_compiler_option/2. This must not affect developers, as the :warnings_as_errors option is managed by Mix tasks, and not directly used via the Code module
[Enumerable] Deprecate returning a two-arity function in Enumerable.slice/1
[List] List.zip/1 is deprecated in favor of Enum.zip/1
[Module] Deprecate Module.eval_quoted/3 in favor of Code.eval_quoted/3
[Range] Deprecate inferring negative ranges on Range.new/2
[Tuple] Tuple.append/2 is deprecated, use Tuple.insert_at/3 instead

Mix

[mix cmd] Deprecate mix cmd --app APP in favor of mix do --app APP
[mix compile] :warnings_as_errors configuration in :elixirc_options is deprecated. Instead pass the --warnings-as-errors flag to mix compile. Alternatively, you might alias the task: aliases: [compile: "compile --warnings-as-errors"]
[mix test] :warnings_as_errors configuration in :test_elixirc_options is deprecated. Instead pass the --warnings-as-errors flag to mix test. Alternatively, you might alias the task: aliases: [test: "test --warnings-as-errors"]
[Mix.Tasks.Compile] Deprecate compilers/0 in favor of Mix.Task.Compiler.compilers/0

v1.17

The CHANGELOG for v1.17 releases can be found in the v1.17 branch.

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