The Dialyzer is a static analysis tool that identifies software discrepancies using a Hindley-Milner style system called Success Typing. It brings some of the benefits of Haskell or Ocaml style type checking to Erlang. If you are not using it as part of the automated build for your system you should be.

Dialyzer does its thing going through all of your direct and transitive dependencies and pulling out lots of information about types, function calls, etc. It then checks that that ‘type universe’ is consistent with the calls you are making to your target. However, All this pulling out of type information takes quite a long time, so Dialyzer has been given the ability to cache this information in files called PLT files.

While PLT files are wonderful for saving you time and you should use them. You must be careful about how you use them. In fact, most people are using them in ways that will cause them problems in the long run. It’s common to have a ‘Global’ PLT file, either shipped with the Erlang distribution or in the users home directory. Using Global PLT files is a lot like using global state in your code. It’s a practice that leads to subtle and hard to debug errors.

Why is that? Well it has a lot to do with the fact that in the post Rebar world version numbers don’t mean what they should. Huh? What does that have to do with Dialyzer? Give me a second, and I will explain. A lot of folks out there have the dependencies in their rebar.config pointing to a branch or other mutable ref instead of a tag. This means that every time they do a rebar get-deps and pull down their deps (maybe after a clean) they have slightly different code under the same version number. That is, the version number is specified in the *.app or the *.app.src, and that doesn’t change but with every new commit the code associated with that version number changes. That pretty much eliminates the value of version numbers as useful identifiers. Unfortunately, Dialyzer and several other OTP tools rely on those version numbers. So let’s take the case where you have a Global PLT file where you keep the cached information about OTP Applications. Internally, this file is organized by application and version. However, every single one of projects you are currently working on you work on (if you depend on branches or head instead tags) are working on subtly different implicit versions for each dependency, all identified by an invalid explicit version that Dialyzer is using.

You may go a long time without this causing you problems, but at some point Dialyzer is going to think your code is wrong based on what it knows about an App but that App is different from the one you have as a dependency and so the warning is invalid. You won’t know that though and you will spend a ton of time trying to figure out what is wrong and eventually give up and ignore the warning in your mind or maybe even stop using Dialyzer.

Fixing The Problem

How do we fix this? We could do this by relying on explicit tags everywhere and making sure that we don’t pollute our version space but that’s hard to do. It’s much simpler and safer to solve this problem by treating the PLT file just like any other compile output. It should be generated on a per project basis and kept in the ‘build’ area of the project. Yes, that means it should be regenerated for each project, and that takes a little while but doing it this way solves all the problems described above in a clean and simple way. You probably already have a Makefile driving Rebar so lets leverage that Makefile to do the Dialyzer magic in simplest and least intrusive way possible. We will set up a couple of tasks to do what we need. The first task is a simple rule that creates the PLT file; the second task runs Dialyzer on the Rebar source.

        - dialyzer --output_plt $(DEPSOLVER_PLT) --build_plt \
                --apps erts kernel stdlib crypto public_key -r deps

dialyzer: $(DEPSOLVER_PLT)
        dialyzer --plt $(DEPSOLVER_PLT) -Wrace_conditions --src src

Another thing to note. Dialyzer will often fail on your dependencies. There are too many fools out there that are not using dialyzer on their project. These rules take that fact of life into account, but you will see error messages from the PLT file creation. You can ignore those; it’s not your responsibility to fix errors in your upstream (though you can and submit them back, it’s always good to be a contributor to your dependencies).

That’s it. You want to change the --apps to a list of actual dependencies for your project rather than the arbitrary list here. A Dialyzer specific example makefile is here. In my next post, I will put up a simple general Makefile that you can use to run a Rebar project. ⤧  Next post Bad Talks And How To Avoid Them ⤧  Previous post Introducing ErlangCamp