Designing Crux – Methods

Crux’s foundations are rooted in the ML language family. Everything is a function. Haskell has this property too, but there is a really sucky aspect of an everything-is-a-function world.


Here’s a common experience I have with Haskell:

let url = getCustomerUrl myCustomerId

OK, I have a string.

I need to check if it starts with “https://”

Scroll to top of file… That means I need to call the startsWith function. What module does it live in again?

Think… is this a Text or a ByteString? Or maybe a String? That determines which module I import…

This is a very common bit of friction, and it comes up all the time in Haskell, both for the standard library and your own code.

Now let’s hypothesize what reading from a data store might feel like in a world with explicit imports:

OK, my React component has an env prop.

I know the UserStore is on the env, but I need to import the Environment module to call getUserStore:

let userStore = Environment.getUserStore(env).

Now I want the list of users, so import UserStore and:

let users = UserStore.getUserList(userStore)

I can see it now: “Why can’t I just write let users = env.getUserStore().getUserList()? Crux sucks!”

In addition, assuming a roughly-one-type-per-module structure, each type gets two “names” that must be used: the module name and the type name. If types are refactored to live in different modules, all the callers have to be updated with new imports. Finally, if those names diverge, it could get confusing.

Dynamic languages like JavaScript and Python solve this usability issue by implementing all method calls with dynamic lookup. Besides the obvious performance hit, this can make analyzing the call graph nontrivial — both by humans and tools.

Languages like C++ and Java and Go take a different approach: they make methods a first-class concept. This adds some complication to the language when you want to use a method as a normal function. C++ has adapters that let you use members as functions sometimes but it’s nicer if methods are just functions.

Crux is not an object-oriented language, but we want the usability benefits of “method syntax”, sometimes known as type-directed name resolution:

  1. with no dynamic dispatch
  2. without any specialize method concept — methods should just be normal functions
  3. and, importantly, while still supporting bidirectional type inference for record types

The reason we have to worry about a conflict with type inference is because records already use dot syntax. Consider the following function:

fun distance(start, end) {
  return sqrt(sqr(end.x - start.x) + sqr(end.y - start.y))

Notice no type annotations! The distance function is inferred to take two records, each of which has floating point x and y properties. It returns a float.

Since we think it’s important that . syntax be used for records, Crux uses an arrow -> for method calls. Method calls only work when the type is known. Fortunately, in most programs, the type inference engine already knows the concrete types of most things. If you try to write a function like:

fun isHTTPSURL(url) {
  return url->startsWith("https://")

you will get a type error, since it doesn’t know what url is. The fix is easy: provide a type annotation on the argument:

fun isHTTPSURL(url: String) {
  return url->startsWith("https://")

In practice, most of the time, the type is known from context. Let’s go back to the Flux UserStore example. With method syntax, it would be spelled:

let users = env->getUserStore()->getUserList()

-> is mainly a way to avoid imports. a->b() can be read as “assuming the type of a is known, look up the symbol b in the module that defined a, and call b(a)”

The env example works because the type of env is known. Therefore, the location of the getUserStore() function is known, and it’s known to return a UserStore, so getUserList is looked up there. No type annotations required, there’s nothing special about member functions, and we achieve all the performance and tooling benefits of static dispatch!

One note: for simplicity, we have been applying the rule that the type must have been inferred before the method call is resolved. Technically, this is more restrictive than necessary. Consider the following example:

fun addSuffix(p) {
  if p->endsWith(".json") {
    return p
  } else {
    return p + ".json"

The p + ".json" expression proves that p’s type is String, and therefore p->endsWith(".json") could be resolved. However, supporting this kind of backwards knowledge transfer would complicate the compiler (I think). Perhaps we’ll look into fixing it later. :)

Another note: sometimes you simply want to reference a “member function” without having a particular instance in mind. You might have the type name in scope, but don’t want to bother importing. We’ve considered support syntax like &String::startsWith to reference the startsWith function in the module that defines the String type. It could be useful in situations like ["foo", "bar", "baz"]->map(&String::length).

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