Deciding when to use the PureScript FFI

I was led to write this because I think we have a bit of problem in the PureScript community with the FFI. It’s very common to see it used, even for things that arguably don’t really need it. By contrast, I’ve been writing Haskell for over 3 years now, and I have never used the Haskell FFI.

Before I start, I want to make it clear that I don’t want you to stop using it entirely other than inside bindings libraries. There are still plenty of situations where it makes sense. I also don’t want you to feel guilty whenever you do decide to use it. Your time is valuable, and I won’t claim to be a better judge of how it should be spent than you.

So. I know the FFI is easy to use, and very attractive for certain problems, particularly when it’s difficult to work out how to give things types in PureScript. But when you feel like you want to use it, I would like to ask you to consider the following:

1. Is there a PureScript library for what I want to do already?

If you’re not sure, Pursuit can be a good place to look. One example of an existing library that doesn’t get used as much as it probably should is unsafe-coerce, which allows you to persuade the type checker that a value of some type is actually some other type.

If there already is a relevant library, but it doesn’t quite work for your use case, then consider opening an issue or sending a pull request. There is a good chance that this will result in improvements to the library that everyone can then benefit from.

2. Could I write this without using the FFI?

If it is possible to write the code you want just in PureScript, without using the FFI, consider doing so. You may well find that avoiding the FFI means that you end up with a better architecture. My impression is that, in languages like JavaScript, the temptation is just too great to mix lots of separate concepts together and throw mutability or other effects in everywhere, and that the antidote is strong, static, expressive types (just like PureScript has).

Of course, if you’re already using PureScript, I probably don’t need to convince you of this. ;)

For example, you may well find that porting a JavaScript library to PureScript gives a better result than trying to write bindings to it via the FFI. I certainly found this to be the case when I wrote ansi.

3. Could I create a separate library for this?

Admittedly, this option is particularly time-consuming. But it does allow you to leverage the community; I think we’re best equipped to design good APIs when we have different people, with different perspectives or use cases, working together. When you create a library, you also create a place which enables this kind of discussion.

Coming up with a typed, purely functional layer on top of stateful or effectful APIs can be hard; if we want to create good bindings, I think the opportunity to get input from other users is just too valuable to pass up.

But why?

In addition to what I’ve already written, there are a few reasons:

Perhaps most importantly, it’s safer. It’s easy to say, “oh, it’s only 5 lines, what can possibly go wrong?” The answer is: lots. While I was porting Pulp to PureScript, the largest time sink by far was me making mistakes such as:

  • using the wrong number of arguments,
  • forgetting to curry,
  • forgetting that I need to return a nullary function for Eff,
  • calling callbacks at the wrong time.

These things, of course, result in runtime errors which are no fun to debug. By segregating FFI code into libraries, these problems only have to be endured once.

It means that we eventually fill gaps in the PureScript ecosystem. PureScript is, of course, still quite young, and has the ecosystem has lots of gaps. After we’ve filled them, we’ll have more time to address more interesting problems.

Finally, it means that it’s easier to use alternative backends. I know of two in development: there’s pure11, which compiles PureScript to C++11, as well as truffled-purescript, which uses Truffle to allow PureScript code to run on the JVM. The more use of the FFI there is across the ecosystem, the harder it is to use other backends.