Elixir v1.14 brings many improvements to the debugging experience in Elixir
and data-type inspection. It also includes a new abstraction for easy
partitioning of processes called PartitionSupervisor, as well as improved
compilation times and error messages.

Elixir v1.14 is the last version to support Erlang/OTP 23. Consider updating
to Erlang/OTP 24 or Erlang/OTP 25.

dbg

Kernel.dbg/2 is a new macro that’s somewhat similar to IO.inspect/2, but
specifically tailored for debugging.

When called, it prints the value of whatever you pass to it, plus the debugged
code itself as well as its location. This code:

# In my_file.exs
feature = %{name: :dbg, inspiration: "Rust"}
dbg(feature)
dbg(Map.put(feature, :in_version, "1.14.0"))

Prints this:

$ elixir my_file.exs
[my_file.exs:2: (file)]
feature #=> %{inspiration: "Rust", name: :dbg}

[my_file.exs:3: (file)]
Map.put(feature, :in_version, "1.14.0") #=> %{in_version: "1.14.0", inspiration: "Rust", name: :dbg}

dbg/2 can do more. It’s a macro, so it understands Elixir code. You can see
that when you pass a series of |> pipes to it. dbg/2 will print the value
for every step of the pipeline. This code:

# In dbg_pipes.exs
__ENV__.file
|> String.split("/", trim: true)
|> List.last()
|> File.exists?()
|> dbg()

Prints this:

$ elixir dbg_pipes.exs
[dbg_pipes.exs:5: (file)]
__ENV__.file #=> "/home/myuser/dbg_pipes.exs"
|> String.split("/", trim: true) #=> ["home", "myuser", "dbg_pipes.exs"]
|> List.last() #=> "dbg_pipes.exs"
|> File.exists?() #=> true

IEx and Prying

dbg/2 supports configurable backends. IEx automatically replaces the default
backend by one that halts the code execution with IEx.Pry, giving developers
the option to access local variables, imports, and more. This also works with
pipelines: if you pass a series of |> pipe calls to dbg (or pipe into it at the
end, like |> dbg()), you’ll be able to step through every line in the pipeline.

You can keep the default behaviour by passing the --no-pry option to IEx.

PartitionSupervisor

PartitionSupervisor is a new module that implements a new supervisor type. The
partition supervisor is designed to help with situations where you have a single
supervised process that becomes a bottleneck. If that process’s state can be
easily partitioned, then you can use PartitionSupervisor to supervise multiple
isolated copies of that process running concurrently, each assigned its own
partition.

For example, imagine you have an ErrorReporter process that you use to report
errors to a monitoring service.

# Application supervisor:
children = [
  # ...,
  ErrorReporter
]

Supervisor.start_link(children, strategy: :one_for_one)

As the concurrency of your application goes up, the ErrorReporter process
might receive requests from many other processes and eventually become a
bottleneck. In a case like this, it could help to spin up multiple copies of the
ErrorReporter process under a PartitionSupervisor.

# Application supervisor
children = [
  {PartitionSupervisor, child_spec: ErrorReporter, name: Reporters}
]

The PartitionSupervisor will spin up a number of processes equal to
System.schedulers_online() by default (most often one per core). Now, when
routing requests to ErrorReporter processes we can use a :via tuple and
route the requests through the partition supervisor.

partitioning_key = self()
ErrorReporter.report({:via, PartitionSupervisor, {Reporters, partitioning_key}}, error)

Using self() as the partitioning key here means that the same process will
always report errors to the same ErrorReporter process, ensuring a form of
back-pressure. You can use any term as the partitioning key.

A Common Example

A common and practical example of a good use case for PartitionSupervisor is
partitioning something like a DynamicSupervisor. When starting many processes
under it, a dynamic supervisor can be a bottleneck, especially if said processes
take a long time to initialize. Instead of starting a single DynamicSupervisor,
you can start multiple:

children = [
  {PartitionSupervisor, child_spec: DynamicSupervisor, name: MyApp.DynamicSupervisors}
]

Supervisor.start_link(children, strategy: :one_for_one)

Now you start processes on the dynamic supervisor for the right partition.
For instance, you can partition by PID, like in the previous example:

DynamicSupervisor.start_child(
  {:via, PartitionSupervisor, {MyApp.DynamicSupervisors, self()}},
  my_child_specification
)

Improved errors on binaries and evaluation

Erlang/OTP 25 improved errors on binary construction and evaluation. These improvements
apply to Elixir as well. Before v1.14, errors when constructing binaries would
often be hard-to-debug generic “argument errors”. With Erlang/OTP 25 and Elixir v1.14,
more detail is provided for easier debugging. This work is part of EEP
54.

Before:

int = 1
bin = "foo"
int <> bin
#=> ** (ArgumentError) argument error

Now:

int = 1
bin = "foo"
int <> bin
#=> ** (ArgumentError) construction of binary failed:
#=>    segment 1 of type 'binary':
#=>    expected a binary but got: 1

Slicing with steps

Elixir v1.12 introduced stepped ranges, which are ranges where you can
specify the “step”:

Enum.to_list(1..10//3)
#=> [1, 4, 7, 10]

Stepped ranges are particularly useful for numerical operations involving
vectors and matrices (see Nx, for example).
However, the Elixir standard library was not making use of stepped ranges in its
APIs. Elixir v1.14 starts to take advantage of steps with support for stepped
ranges in a couple of functions. One of them is Enum.slice/2:

letters = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]
Enum.slice(letters, 0..5//2)
#=> ["a", "c", "e"]

binary_slice/2 (and binary_slice/3 for completeness) has been added to the
Kernel module, that works with bytes and also support stepped ranges:

binary_slice("Elixir", 1..5//2)
#=> "lx"

Expression-based inspection and Inspect improvements

In Elixir, it’s conventional to implement the Inspect protocol for opaque
structs so that they’re inspected with a special notation, resembling this:

MapSet.new([:apple, :banana])
#MapSet<[:apple, :banana]>

This is generally done when the struct content or part of it is private and the
%name{...} representation would reveal fields that are not part of the public
API.

The downside of the #name<...> convention is that the inspected output is not
valid Elixir code. For example, you cannot copy the inspected output and paste
it into an IEx session.

Elixir v1.14 changes the convention for some of the standard-library structs.
The Inspect implementation for those structs now returns a string with a valid
Elixir expression that recreates the struct when evaluated. In the MapSet
example above, this is what we have now:

fruits = MapSet.new([:apple, :banana])
MapSet.put(fruits, :pear)
#=> MapSet.new([:apple, :banana, :pear])

The MapSet.new/1 expression evaluates to exactly the struct that we’re
inspecting. This allows us to hide the internals of MapSet, while keeping
it as valid Elixir code. This expression-based inspection has been
implemented for Version.Requirement, MapSet, and Date.Range.

Finally, we have improved the Inspect protocol for structs so that
fields are inspected in the order they are declared in defstruct.
The option :optional has also been added when deriving the Inspect
protocol, giving developers more control over the struct representation.
See the updated documentation for Inspect for a general rundown on
the approaches and options available.

v1.14.0-rc.0 (2022-08-01)

1. Enhancements

EEx

• [EEx] Support multi-line comments to EEx via <%!-- --%>
• [EEx] Add EEx.tokenize/2

Elixir

• [Access] Add Access.slice/1
• [Application] Add Application.compile_env/4 and Application.compile_env!/3 to read the compile-time environment inside macros
• [Calendar] Support ISO8601 basic format parsing with DateTime.from_iso8601/2
• [Calendar] Add day/hour/minute on add/diff across different calendar modules
• [Code] Add :normalize_bitstring_modifiers to Code.format_string!/2
• [Code] Emit deprecation and type warnings for invalid options in on Code.compile_string/2 and Code.compile_quoted/2
• [Code] Warn if an outdated lexical tracker is given on eval
• [Code] Add Code.env_for_eval/1 and Code.eval_quoted_with_env/3
• [Code] Improve stacktraces from eval operations on Erlang/OTP 25+
• [Code.Fragment] Add support for __MODULE__ in several functions
• [Code.Fragment] Support surround and context suggestions across multiple lines
• [Enum] Allow slicing with steps in Enum.slice/2
• [File] Support dereference_symlinks: true in File.cp/3 and File.cp_r/3
• [Float] Do not show floats in scientific notation if below 1.0e16 and the fractional value is precisely zero
• [Float] Add Float.min_finite/0 and Float.max_finite/0
• [Inspect] Improve error reporting when there is a faulty implementation of the Inspect protocol
• [Inspect] Allow :optional when deriving the Inspect protocol for hiding fields that match their default value
• [Inspect] Inspect struct fields in the order they are declared in defstruct
• [Inspect] Use expression-based inspection for Date.Range, MapSet, and Version.Requirement
• [IO] Support Macro.Env and keywords as stacktrace definitions in IO.warn/2
• [IO] Add IO.ANSI.syntax_colors/0 and related configuration to be shared across IEx and dbg
• [Kernel] Add new dbg/0-2 macro
• [Kernel] Allow any guard expression as the size of a bitstring in a pattern match
• [Kernel] Allow composite types with pins as the map key in a pattern match
• [Kernel] Print escaped version of control chars when they show up as unexpected tokens
• [Kernel] Warn on confusable non-ASCII identifiers
• [Kernel] Add .. as a nullary operator that returns 0..-1//1
• [Kernel] Implement Unicode Technical Standard #39 recommendations. In particular, we warn for confusable scripts and restrict identifiers to single-scripts or highly restrictive mixed-scripts
• [Kernel] Automatically perform NFC conversion of identifiers
• [Kernel] Add binary_slice/2 and binary_slice/3
• [Kernel] Lazily expand module attributes to avoid compile-time deps
• [Kernel] Automatically cascade generated: true annotations on macro expansion
• [Keyword] Add Keyword.from_keys/2 and Keyword.replace_lazy/3
• [List] Add List.keysort/3 with support for a sorter function
• [Macro] Add Macro.classify_atom/1 and Macro.inspect_atom/2
• [Macro] Add Macro.expand_literal/2 and Macro.path/2
• [Macro.Env] Add Macro.Env.prune_compile_info/1
• [Map] Add Map.from_keys/2 and Map.replace_lazy/3
• [MapSet] Add MapSet.filter/2, MapSet.reject/2, and MapSet.symmetric_difference/2
• [Node] Add Node.spawn_monitor/2 and Node.spawn_monitor/4
• [Module] Support new @after_verify attribute for executing code whenever a module is verified
• [PartitionSupervisor] Add PartitionSupervisor that starts multiple isolated partitions of the same child for scalability
• [Path] Add Path.safe_relative/1 and Path.safe_relative_to/2
• [Registry] Add Registry.count_select/2
• [Stream] Add Stream.duplicate/2 and Stream.transform/5
• [String] Support empty lookup lists in String.replace/3, String.split/3, and String.splitter/3
• [String] Allow slicing with steps in String.slice/2
• [Task] Add :zip_input_on_exit option to Task.async_stream/3
• [Task] Store :mfa in the Task struct for reflection purposes
• [URI] Add URI.append_query/2
• [Version] Add Version.to_string/1
• [Version] Colorize Version.Requirement source in the Inspect protocol

ExUnit

• [ExUnit] Add ExUnit.Callbacks.start_link_supervised!/2
• [ExUnit] Add ExUnit.run/1 to rerun test modules
• [ExUnit] Colorize summary in yellow with message when all tests are excluded
• [ExUnit] Display friendly error when test name is too long

IEx

• [IEx] Evaluate --dot-iex line by line
• [IEx] Add line-by-line evaluation of IEx breakpoints
• [IEx.Autocomplete] Autocomplete bitstrings modifiers (after :: inside <<...>>)
• [IEx.Helpers] Allow an atom to be given to pid/1

Logger

• [Logger] Add Logger.put_process_level/2

Mix

• [mix compile] Add --no-optional-deps to skip optional dependencies to test compilation works without optional dependencies
• [mix compile] Include column information on error diagnostics when possible
• [mix deps] Mix.Dep.Converger now tells which deps formed a cycle
• [mix do] Support --app option to restrict recursive tasks in umbrella projects
• [mix do] Allow using + as a task separator instead of comma
• [mix format] Support filename in mix format - when reading from stdin
• [mix format] Compile if mix format plugins are missing
• [mix new] Do not allow projects to be created with application names that conflict with multi-arg Erlang VM switches
• [mix profile] Return the return value of the profiled function
• [mix release] Make BEAM compression opt-in
• [mix release] Let :runtime_config_path accept false to skip the config/runtime.exs
• [mix test] Improve error message when suite fails due to coverage
• [mix test] Support :test_elixirc_options and default to not generating docs nor debug info chunk for tests
• [mix xref] Support --group flag in mix xref graph

2. Bug fixes

Elixir

• [Calendar] Handle widths with “0” in them in Calendar.strftime/3
• [CLI] Improve errors on incorrect --rpc-eval usage
• [CLI] Return proper exit code on Windows
• [Code] Do not emit warnings when formatting code
• [Enum] Allow slices to overflow on both starting and ending positions
• [Kernel] Do not allow restricted characters in identifiers according to UTS39
• [Kernel] Define __exception__ field as true when expanding exceptions in typespecs
• [Kernel] Warn if any of True, False, and Nil aliases are used
• [Kernel] Warn on underived @derive attributes
• [Kernel] Remove compile-time dependency from defimpl :for
• [Kernel] Track all arities on imported functions
• [Protocol] Warn if a protocol has no definitions
• [Regex] Show list options when inspecting a Regex manually defined with Regex.compile/2
• [String] Allow slices to overflow on both starting and ending positions

ExUnit

• [ExUnit] Do not crash when diffing unknown bindings in guards
• [ExUnit] Properly print diffs when comparing improper lists with strings at the tail position
• [ExUnit] Add short hash to tmp_dir in ExUnit to avoid test name collision
• [ExUnit] Do not store logs in the CLI formatter (this reduces memory usage for suites with capture_log)
• [ExUnit] Run ExUnit.after_suite/1 callback even when no tests run
• [ExUnit] Fix scenario where setup with imported function from within describe failed to compile

IEx

• [IEx] Disallow short-hand pipe after matches
• [IEx] Fix exports/1 in IEx for long function names

Mix

• [mix compile.elixir] Fix --warnings-as-errors when used with --all-warnings
• [mix compile.elixir] Ensure semantic recompilation cascades to path dependencies
• [mix compile.elixir] Lock the compiler to avoid concurrent usage
• [mix format] Do not add new lines if the formatted file is empty
• [mix release] Only set RELEASE_MODE after env.{sh,bat} are executed
• [mix release] Allow application mode configuration to cascade to dependencies
• [mix xref] Do not emit already consolidated warnings during mix xref trace
• [Mix] Do not start apps with runtime: false on Mix.install/2

3. Soft deprecations (no warnings emitted)

Elixir

• [File] Passing a callback as third argument to File.cp/3 and File.cp_r/3 is deprecated.
  Instead pass the callback the :on_conflict key of a keyword list

EEx

• [EEx] Using <%# ... %> for comments is deprecated. Please use <% # ... %> or the new multi-line comments with <%!-- ... --%>

Logger

• [Logger] Deprecate Logger.enable/1 and Logger.disable/1 in favor of Logger.put_process_level/2

Mix

• [mix cmd] The --app option in mix cmd CMD is deprecated in favor of the more efficient mix do --app app cmd CMD

4. Hard deprecations

Elixir

• [Application] Calling Application.get_env/3 and friends in the module body is now discouraged, use Application.compile_env/3 instead
• [Bitwise] use Bitwise is deprecated, use import Bitwise instead
• [Bitwise] ~~~ is deprecated in favor of bnot for clarity
• [Kernel.ParallelCompiler] Returning a list or two-element tuple from :each_cycle is deprecated, return a {:compile | :runtime, modules, warnings} tuple instead
• [Kernel] Deprecate the operator <|> to avoid ambiguity with upcoming extended numerical operators
• [String] Deprecate passing a binary compiled pattern to String.starts_with?/2

Logger

• [Logger] Deprecate $levelpad on message formatting

Mix

• [Mix] Mix.Tasks.Xref.calls/1 is deprecated in favor of compilation tracers

5. Backwards incompatible changes

Mix

• [mix local.rebar] Remove support for rebar2, which has not been updated in 5 years, and is no longer supported on recent Erlang/OTP versions