# Run Dialyzer on Elixir on Travis CI

In my last post, I talked about Elixir’s typespec annotations and Erlang’s static analysis tool, Dialyzer. All that talk was great and all, but how do you actually use Dialyzer on Elixir projects?

In particular, how do you run Dialyzer on an Elixir project on Travis CI? In today’s blog post, I’ll show you how.

## Persistent Lookup Table

Dialyzer requires a “persistent lookup table”, or PLT, in order to perform its analysis. This PLT holds references to all the system Erlang libraries, and Elixir’s source code itself. In order to run Dialyzer on an Elixir project, you’ve got to first generate a PLT for your system.

Yes, the PLT has to be generated for the system that you’re going to run Dialyzer on, because the paths in the PLT are hard coded when it is generated. A PLT generated on your Mac won’t work on Travis.

What’s more, the PLT takes quite a while to generate. If you generated it every time you ran your build on Travis, you’d slow down your builds by minutes.

Luckily for you, I’ve built a bunch of PLTs for Elixir on Travis already. You can find them here: danielberkompas/travis_elixir_plts. You’re welcome.

## .travis.yml

With a prebuilt PLT, running dialyzer on Travis is simple:

language: elixir
otp_release:
- 17.4
before_script:
- export PLT_FILENAME=elixir-${TRAVIS_ELIXIR_VERSION}_${TRAVIS_OTP_RELEASE}.plt
- export PLT_LOCATION=/home/travis/$PLT_FILENAME # Download PLT from danielberkompas/travis_elixir_plts on Github # Store in$PLT_LOCATION
- wget -O $PLT_LOCATION https://raw.github.com/danielberkompas/travis_elixir_plts/master/$PLT_FILENAME
script:
- mix test
- dialyzer --no_check_plt --plt $PLT_LOCATION --no_native _build/test/lib/$YOUR_PROJECT_NAME/ebin

A few notes on the options passed to Dialyzer here:

• --no_check_plt: Increases speed. Dialyzer assumes PLT is up to date.
• --no_native: Also increases speed. Because it is used to analyze large projects, Dialyzer likes to compile parts into native code before doing analysis. This takes time.
• _build/test/lib/$YOUR_PROJECT_NAME/ebin: The directory in which the compiled BEAM files created by Mix are. Replace$YOUR_PROJECT_NAME with whatever your folder name happens to be.

Obviously, you could pass any options supported by Dialyzer here. Run dialyzer --help to see them all. And that’s that! You can now ensure that both you and all your collaborators pay attention to Dialyzer.