Mix tasks to simplify use of Dialyzer in Elixir projects.

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Mix tasks to simplify use of Dialyzer in Elixir projects.


Dialyxir is available on hex.pm.

To add it to a mix project, just add a line like this in your deps function in mix.exs:

defp deps do
    {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false},
mix do deps.get, deps.compile


Use dialyxir from the directory of the mix project you want to analyze; a PLT file will be created or updated if required and the project will be automatically compiled.

mix dialyzer

Command line options

  • --no-compile - do not compile even if needed.
  • --no-check - do not perform (quick) check to see if PLT needs to be updated.
  • --ignore-exit-status - display warnings but do not halt the VM or return an exit status code.
  • --format short - format the warnings in a compact format, suitable for ignore file using Elixir term format.
  • --format raw - format the warnings in format returned before Dialyzer formatting.
  • --format dialyxir - format the warnings in a pretty printed format. (default)
  • --format dialyzer - format the warnings in the original Dialyzer format, suitable for ignore file using simple string matches.
  • --format github - format the warnings in the Github Actions message format.
  • --format ignore_file - format the warnings in {file, warning} format for Elixir Format ignore file.
  • --format ignore_file_strict - format the warnings in {file, short_description} format for Elixir Format ignore file.
  • --quiet - suppress all informational messages.

Warning flags passed to this task are passed on to :dialyzer - e.g.

mix dialyzer --unmatched_returns

There is information available about the warnings via the explain task - e.g.

mix dialyzer.explain unmatched_return

If invoked without arguments, mix dialyzer.explain will list all the known warnings.

Continuous Integration

To use Dialyzer in CI, you must be aware of several things:

  1. Building the project-level PLT file may take a while if a project has many dependencies.
  2. The project-level PLT should be cached using the CI caching system.
  3. Optional: The core Erlang/Elixir PLT could also be cached in CI.
  4. The PLT will need to be rebuilt whenever adding a new Erlang or Elixir version to your build.
# mix.exs
def project do
    dialyzer: [
      # Put the project-level PLT in the priv/ directory (instead of the default _build/ location)
      plt_file: {:no_warn, "priv/plts/project.plt"}

      # The above is equivalent to:
      # plt_local_path: "priv/plts/project.plt"

      # You could also put the core Erlang/Elixir PLT into the priv/ directory like so:
      # plt_core_path: "priv/plts/core.plt"
# .gitignore

Example CI Configs

With Explaining Stuff

Dialyzer is a static analysis tool for Erlang and other languages that compile to BEAM bytecode for the Erlang VM. It can analyze the BEAM files and provide warnings about problems in your code including type mismatches and other issues that are commonly detected by static language compilers. The analysis can be improved by inclusion of type hints (called specs) but it can be useful even without those. For more information I highly recommend the Success Typings paper that describes the theory behind the tool.

Usage is straightforward but you should be aware of the available configuration settings you may wish to add to your mix.exs file.


The Persistent Lookup Table (PLT) is basically a cached output of the analysis. This is important because you'd probably stab yourself in the eye with a fork if you had to wait for Dialyzer to analyze all the standard library and OTP modules you are using every time you ran it. Running the mix task dialyzer by default builds several PLT files:

  • A core Erlang file in $MIX_HOME/dialyxir_erlang-$OTP_VERSION.plt
  • A core Elixir file in $MIX_HOME/dialyxir_erlang-$OTP_VERSION-$ELIXIR_VERSION.plt
  • A project environment specific file in _build/$MIX_ENV/dialyze_erlang-$OTP_VERSION_elixir-$ELIXIR_VERSION_deps-$MIX_ENV.plt

The core files are simply copied to your project folder when you run dialyxir for the first time with a given version of Erlang and Elixir. By default, all the modules in the project PLT are checked against your dependencies to be sure they are up to date. If you do not want to use MIX_HOME to store your core Erlang and Elixir files, you can provide a :plt_core_path key with a file path. You can specify a different directory for the project PLT file with the :plt_local_path keyword.

You can also specify a different filename for the project PLT file with the :plt_file keyword option. This is deprecated for local use, but fine to use in CI. The reason for the local deprecation is that people were using it with older versions of dialyxir to have project-specific PLTs, which are now the default. To silence the deprecation warning in CI, specify this value as plt_file: {:no_warn, "/myproject/mypltfile"}.

The core PLTs include a basic set of OTP applications, as well as all of the Elixir standard libraries. The apps included by default are [:erts, :kernel, :stdlib, :crypto].

If you don't want to include the default apps you can specify a :plt_apps key and list there only the apps you want in the PLT. Using this option will mean dependencies are not added automatically (see below). If you want to just add an application to the list of defaults and dependencies you can use the :plt_add_apps key.

If you want to ignore a specific dependency, you can specify it in the :plt_ignore_apps key.


OTP application dependencies are (transitively) added to your PLT by default. The applications added are the same as you would see displayed with the command mix app.tree. There is also a :plt_add_deps option you can set to control the dependencies added. The following options are supported:

  • :apps_direct - Only Direct OTP runtime application dependencies - not the entire tree
  • :app_tree - Transitive OTP runtime application dependencies e.g. mix app.tree (default)

The example below changes the default to include only direct OTP dependencies, adds another specific dependency, and removes a dependency from the list. This can be helpful if a large dependency tree is creating memory issues and only some of the transitive dependencies are required for analysis.

def project do
    app: :my_app,
    version: "0.0.1",
    deps: deps,
    dialyzer: [
      plt_add_deps: :apps_direct,
      plt_add_apps: [:wx],
      plt_ignore_apps: [:mnesia]


Explanations are available for classes of warnings by executing mix dialyzer.explain warning_name. It will include a description about the type of warning, as well as a small example that would also cause that warning. Poor explanations and examples should be considered issues in this library, and pull requests are very welcome! The warning name is returned from the --format short and --format dialyzer flags. List available warnings with mix dialyzer.explain.


Dialyxir supports formatting the errors in several different ways:

  • Short - By passing --format short, the structs and other spec/type information will be dropped from the error message, with a minimal message. This is useful for CI environments. Includes warning_name for use in explanations.
  • Dialyzer - By passing --format dialyzer, the messages will be printed in the default Dialyzer format. This format is used in legacy string matching ignore files.
  • Raw - By passing --format raw, messages will be printed in their form before being pretty printed by Dialyzer or Dialyxir.
  • Dialyxir (default) -- By passing --format dialyxir, messages will be converted to Elixir style messages then pretty printed and formatted. Includes warning_name for use in explanations.


Dialyzer supports a number of warning flags used to enable or disable certain kinds of analysis features. Until version 0.4, dialyxir used by default the additional warning flags shown in the example below. However some of these create warnings that are often more confusing than helpful, particularly to new users of Dialyzer. As of 0.4, there are no longer any flags used by default. To get the old behavior, specify them in your Mix project file. For compatibility reasons you can use either the -Wwarning convention of the dialyzer CLI, or (preferred) the WarnOpts atoms supported by the API. e.g.

def project do
    app: :my_app,
    version: "0.0.1",
    deps: deps,
    dialyzer: [flags: ["-Wunmatched_returns", :error_handling, :underspecs]]


By default only the ebin in the _build directory for the current mix environment of your project is included in paths to search for BEAM files to perform analysis on. You can specify a list of locations to find BEAMS for analysis with :paths keyword.

def project do
    app: :my_app,
    version: "0.0.1",
    deps: deps,
    dialyzer: [
      plt_add_apps: [:mnesia],
      flags: [:unmatched_returns, :error_handling, :no_opaque],
      paths: ["_build/dev/lib/my_app/ebin", "_build/dev/lib/foo/ebin"]

Ignore Warnings

Dialyxir defaults

By default dialyxir has always included the :unknown warning option so that warnings about unknown functions are returned. This is usually a clue that the PLT is not complete and it may be best to leave it on, but it can be disabled entirely by specifying remove_defaults: [:unknown] in your config.

A better option is to ignore the specific warnings you can't fix (maybe due to a bug upstream, or a dependency you just don't want to include in your PLT due to time/memory in building the PLT file.)

Module attribute

Dialyzer has a built-in support for ignoring warnings through a @dialyzer module attribute. For example:

defmodule Myapp.Repo do
  use Ecto.Repo, otp_app: :myapp
  @dialyzer {:nowarn_function, rollback: 1}

More details can be found in the erlang documentation

Ignore file

If you want to ignore well-known warnings, you can specify a file path in :ignore_warnings.

def project do
    app: :my_app,
    version: "0.0.1",
    deps: deps,
    dialyzer: [ignore_warnings: "dialyzer.ignore-warnings"]

This file comes in two formats: --format dialyzer string matches (compatible with <= 0.5.1 ignore files), and the term format.

Dialyzer will look for an ignore file using the term format with the name .dialyzer_ignore.exs by default if you don't specify something otherwise.

Simple String Matches

Any line of dialyzer format output (partially) matching a line in "dialyzer.ignore-warnings" is filtered.

Note that copying output in the default format will not work! Run mix dialyzer --format dialyzer to produce output suitable for the ignore file.

For example, in a project where mix dialyzer --format dialyzer outputs:

  Proceeding with analysis...
config.ex:64: The call ets:insert('Elixir.MyApp.Config',{'Elixir.MyApp.Config',_}) might have an unintended effect due to a possible race condition caused by its combination with the ets:lookup('Elixir.MyApp.Config','Elixir.MyApp.Config') call in config.ex on line 26
config.ex:79: Guard test is_binary(_@5::#{'__exception__':='true', '__struct__':=_, _=>_}) can never succeed
config.ex:79: Guard test is_atom(_@6::#{'__exception__':='true', '__struct__':=_, _=>_}) can never succeed
 done in 0m1.32s
done (warnings were emitted)

If you wanted to ignore the last two warnings about guard tests, you could add to dialyzer.ignore-warnings:

Guard test is_binary(_@5::#{'__exception__':='true', '__struct__':=_, _=>_}) can never succeed
Guard test is_atom(_@6::#{'__exception__':='true', '__struct__':=_, _=>_}) can never succeed

And then run mix dialyzer would output:

  Proceeding with analysis...
config.ex:64: The call ets:insert('Elixir.MyApp.Config',{'Elixir.MyApp.Config',_}) might have an unintended effect due to a possible race condition caused by its combination with the ets:lookup('Elixir.MyApp.Config','Elixir.MyApp.Config') call in config.ex on line 26
 done in 0m1.32s
done (warnings were emitted)

Elixir Term Format

Dialyxir also recognizes an Elixir format of the ignore file. If your ignore file is an exs file, Dialyxir will evaluate it and process its data structure. A line may be either a tuple or an arbitrary Regex applied to the short-description format of Dialyzer output (mix dialyzer --format short). The file looks like the following:

# .dialyzer_ignore.exs
  # {short_description}
  {":0:unknown_function Function :erl_types.t_is_opaque/1/1 does not exist."},
  # {short_description, warning_type}
  {":0:unknown_function Function :erl_types.t_to_string/1 does not exist.", :unknown_function},
  # {short_description, warning_type, line}
  {":0:unknown_function Function :erl_types.t_to_string/1 does not exist.", :unknown_function, 0},
  # {file, warning_type, line}
  {"lib/dialyxir/pretty_print.ex", :no_return, 100},
  # {file, warning_description}
  {"lib/dialyxir/warning_helpers.ex", "Function :erl_types.t_to_string/1 does not exist."},
  # {file, warning_type}
  {"lib/dialyxir/warning_helpers.ex", :no_return},
  # {file}
  # regex
  ~r/my_file\.ex.*my_function.*no local return/

Note that short_description contains additional information that warning_description does not.

Entries for existing warnings can be generated with one of the following:

  • mix dialyzer --format ignore_file
  • mix dialyzer --format ignore_file_strict (recommended)

For example, if mix dialyzer --format short gives you a result like:

lib/something.ex:15:no_return Function init/1 has no local return.
lib/something.ex:36:no_return Function refresh/0 has no local return.
lib/something.ex:45:no_return Function create/2 has no local return.
lib/something.ex:26:no_return Function update/2 has no local return.
lib/something.ex:49:no_return Function delete/1 has no local return.

If you had used --format ignore_file, you'd be given a single file ignore line for all five warnings:

# .dialyzer_ignore.exs
  # {file, warning_type}
  {"lib/something.ex", :no_return},

If you had used --format ignore_file_strict, you'd be given more granular ignore lines:

# .dialyzer_ignore.exs
  # {file, warning_description}
  {"lib/something.ex", "Function init/1 has no local return."},
  {"lib/something.ex", "Function refresh/0 has no local return."},
  {"lib/something.ex", "Function create/2 has no local return."},
  {"lib/something.ex", "Function update/2 has no local return."},
  {"lib/something.ex", "Function delete/1 has no local return."},

List unused Filters

As filters tend to become obsolete (either because a discrepancy was fixed, or because the location for which a filter is needed changes), listing unused filters might be useful. This can be done by setting the :list_unused_filters option to true in mix.exs. For example:

dialyzer: [
  ignore_warnings: "ignore_test.exs",
  list_unused_filters: true

This option can also be set on the command line with --list-unused-filters. When used without --ignore-exit-status, this option will result in an error status code.

no_umbrella flag

Projects with lockfiles at a parent folder are treated as umbrella projects. In some cases however you may wish to have the lockfile on a parent folder without having an umbrella. By setting the no_umbrella flag to true your project will be treated as a non umbrella project:

dialyzer: [
  no_umbrella: true