React Native, PureScript, And TS-Bridge: Overcoming Import Issues

Hey there, fellow developers! If you're diving into the exciting world of React Native development and exploring the power of PureScript with the ts-bridge library, you might stumble upon some interesting challenges, especially when it comes to how your code gets bundled and optimized. Today, we're going to tackle a specific hurdle related to the `tsValues` helper in purescript-ts-bridge and how it interacts with React Native's build process. We'll explore why it happens, walk through a clever workaround, and even touch upon some advanced concepts for dead code elimination. So, grab your favorite beverage, and let's get started on this optimization adventure!

The `tsValues` Conundrum in React Native Bundling

Let's talk about the core issue. When you use the tsValues helper in purescript-ts-bridge, its primary purpose is to generate a TypeScript definition (a proxy) for your exported PureScript types. You provide a value, and the library uses that to infer the type structure. Now, here's the kicker: even if you never actually *use* this proxy value in your runtime JavaScript, the bundler still imports and includes it. This creates an unintended dependency, forcing your main script to carry the baggage of whatever the original `tsValues` argument depended on. In the context of React Native, which traditionally uses Flow and Babel for its JavaScript transformations, this can lead to unexpected build errors or bloated bundles because the compiled PureScript code might be trying to include modules that aren't relevant or compatible with the React Native environment. It's like bringing a whole toolbox to a job where you only needed a single screwdriver – unnecessary weight and potential complications. This optimization problem isn't unique to PureScript; it's a general challenge in complex JavaScript ecosystems where build tools and bundlers need to be smart enough to identify and discard unused code. The goal is to ensure that only the absolutely necessary code makes it into the final bundle, leading to faster load times and a more efficient application. This requires a deep understanding of both the programming language's features and the bundler's capabilities, and sometimes, a bit of creative problem-solving to bridge the gap between them. We'll explore how this plays out specifically within the React Native environment, where build configurations can be particularly intricate.

A Workaround Using `spago bundle` and Mocking

To overcome this import issue, we can employ a neat trick using Spago, the build tool for PureScript. The strategy involves telling the bundler to treat the problematic `react-native` dependency as an empty module during the bundling process. This effectively fools the build system into thinking `react-native` is available and has the expected exports (which are none in this case), but it prevents the actual `react-native` code from being pulled into your PureScript bundle. Here's how you can do it:

spago bundle --platform node \
  --bundle-type app \
  --bundler-args --alias:react-native=./rn-mock.js

In this command, --platform node is specified because we're essentially tricking the bundler into thinking it's building for a Node.js environment where such aliasing is more straightforward. The crucial part is --bundler-args --alias:react-native=./rn-mock.js. This tells the underlying bundler (like Webpack or esbuild, depending on your Spago configuration) to replace any import of `react-native` with the contents of a local file named `rn-mock.js`. You'll need to create this `rn-mock.js` file in the same directory or provide a correct path. Inside `rn-mock.js`, you simply export an empty object or whatever minimal structure is required to satisfy the import:

export default {};

By doing this, the PureScript code that depends on `react-native` (perhaps indirectly through `tsValues`) will still resolve, but it won't pull in any actual `react-native` code. This bypasses the dead code elimination problem for the `tsValues` argument, ensuring that your bundle remains lean and only includes what's truly necessary for your React Native application. This technique is particularly useful when dealing with libraries that have environment-specific dependencies that you don't intend to use at runtime from your PureScript code, but are required by the build process. It’s a form of dependency injection or mocking at the build level, which can save a lot of headaches when integrating different technologies. Remember to adjust the path to `rn-mock.js` as needed based on your project structure. Experimentation is key here, as build systems can be finicky, but this approach has proven effective in similar scenarios.

Exploring Advanced Optimization with `purescript-backend-optimizer`

For those who like to dig deeper into optimization, the PureScript backend optimizer (purescript-backend-optimizer) is a powerful tool. While it also faced a similar issue initially, we found that strategically placing the proxy resolution within the current module, rather than a dependent one, allowed the inlining to kick in effectively. This means the optimizer could see that the value used for `tsValues` was only needed for its type and could be inlined directly, avoiding the external import altogether. Let's look at the code structure that facilitates this:

proxy :: forall a. a -> Proxy a
proxy _ = Proxy

exports :: Either TSB.AppError (Array DTS.TsModuleFile)
exports = TSB.tsModuleFile "Components" [
  TSB.tsValues TSB.Tok { counter: ExportedComponent counter }
]

In this snippet, TSB.tsValues TSB.Tok { counter: ExportedComponent counter } is the key. By defining the structure directly within the `exports` definition, we help the optimizer understand the context. The proxy function here is a simple identity function that takes a value and returns a `Proxy` type. When used with `tsValues`, it allows us to pass a value whose type we want to expose, without necessarily needing to execute the value itself at runtime. The optimization happens because the optimizer can see that `counter` is only used to determine the type structure for the TypeScript export. If `counter` itself were a complex import or a large piece of code, this technique would prevent that entire dependency from being bundled. This is a prime example of how code structure and placement can significantly impact compiler optimizations. Furthermore, if you're looking for more fine-grained control, you might explore inline directives within PureScript. These directives can provide hints to the compiler about which functions or values should be inlined, potentially improving performance and helping with dead code elimination. While purescript-backend-optimizer is a fantastic tool, understanding its capabilities and how to structure your code to best leverage its optimizations is an ongoing process. It's about working with the optimizer, not against it, to achieve the most efficient and performant code possible. Keep experimenting with different code layouts and compiler flags to see what yields the best results for your specific project needs.

Thinking About Dead Code Elimination and Type-Level Programming

A more advanced thought revolves around simulating constructs like typeof x in JavaScript, which allows you to reference the *type* of a value without depending on the value *itself*. This is the holy grail for truly effective dead code elimination. Imagine defining your types in a separate PureScript module and then having both your implementation code and your ts-bridge generation code depend on that shared type module. This approach, while potentially more robust, does come with its own set of challenges, primarily around maintaining consistency and avoiding errors. If the type definition in the shared module drifts from the actual implementation, your generated TypeScript will be incorrect, leading to runtime errors.

The core question is: can we get the type of a value in PureScript without actually referencing the value's runtime representation? This is where type-level programming and advanced compiler features come into play. PureScript's powerful type system allows for sophisticated manipulations, but effectively decoupling type information from runtime values in a way that bundlers can fully optimize is tricky. One potential avenue could be exploring template literal types or similar features in TypeScript, if you're generating TypeScript definitions. However, the challenge lies in the PureScript side first. Perhaps there could be a specific