Why Purescript?

One thing that I feel Haskell doesn’t do well presently is interface with GUI libraries. One issue is that, because most such libraries are largely stateful, there’s some impedance when trying to adopt them into idiomatic Haskell code. Another problem altogether is trying to get cross-platform GUI libraries to even build/run. It’s a problem I’ve written about before.

I’ve had some good success in the past with using Haskell libraries that leverage the browser as a robust, cross-platform GUI. However, running the GUI in the browser and the controller logic in Haskell is problematic for all but the simplest GUIs for a few reasons:

  1. The round trip communication between the browser and Haskell code introduces latency that might be unacceptably high. Indeed, some typical operations might require several round trips.
  2. More and more, the latest GUI component libraries are written in Javascript, so using GUI components that are designed with Javascript in mind is more difficult with a Haskell backend.
  3. Keeping the GUI in sync with the model requires fragile DOM updates. Smarter, more functional and performant approaches like virtual-dom don’t integrate well with Haskell backends because the entire model or virtual DOM tree must be serialized to the browser for each GUI update.
  4. More and more, browser technologies are becoming the basis for cross-platform apps.

But here’s the thing: writing Javascript is no fun. Haskell is fun. Can we write Haskell but get the advantages of Javascript?

Haskell to Javascript Compilers

Write Haskell. Compile to Javascript. Run anywhere. Sounds good, right? Some projects are developing exactly this kind of solution. Each has its advantages. For example, Fay has a nice FFI to bind to Javascript libraries and invoke custom Javascript code. Haste reportedly generates fast, understandable Javascript. GHCJS supposedly compiles any Haskell code with ease.

However, I think all these solutions suffer from a key deficiency: Haskell baggage. Each compiles Haskell syntax and supports Haskell semantics, including laziness. Usually, this necessitates a large Javascript runtime be packaged with the generated Javascript program just to execute it in a way Haskell would. After all, they are the forcing square peg of Javascript into the round hole of Haskell.

Purescript – A Different Approach

Purescript is a Haskell-like language that compiles to Javascript. Unlike the above approaches that accommodate all/most of Haskell’s syntax and features, Purescript is a functional language designed from the ground up to target Javascript, and its syntax is close enough to Haskell that it should be easy for any Haskeller to pick up quickly. It features:

  1. No burdensome runtime.
  2. Strict, Javascript-like evaluation.
  3. Support for Javascript object literal notation.
  4. A type system that is arguably more powerful and/or more convenient than Haskell’s.
  5. A dead simple FFI that makes Javascript library interop relatively painless.
  6. Other cool goodies.

In a future post, I’ll explore these advantages more in depth, but here I wanted to describe what might motivate a Haskeller (or anyone) to give Purescript a try.