[IceDane]

The problem with your counter-argument is that it hinges on a false premise: That you need or even want a function like `assoc` which is polymorphic over everything. It's an extremely overloaded function which does a lot of things at once, and in many circles and arguably in general within the realm of software design, this is considered a smell.

In practice, what you want is something that allows you to do this safely for the concrete type you're working with. If you want an abstraction that covers all of it, there are ways to achieve this in a type-safe manner, such as traits/type classes. Even in clojure, you're not working with everything at once all the time. You are working with a record, or a vector, or whatever. The fact that you can use one function for all of them is mostly just needless cleverness. In Clojure, you have to keep the type of the data you are working with in your head at all times, because even though `assoc` "just works" for many cases, that's not true in all cases. It will happily insert an integer key into a record without issue, which may or may not be waht you want. But you can also try to insert an atom key into a vector, which then crashes loudly. This is clearly an asymmetry in the abstraction.

Moreover, pointing out that Haskell cannot do what you'd want to do in this case doesn't make a lot of sense. I mentioned Haskell precisely because its type system is extremely powerful and complicated to understand for a lot of people, but still doesn't achieve the kind of flexibility we are looking for - it lacks row polymorphism.

To answer your actual question: Typing a function like that for the individual cases is bordering on trivial in a language such as typescript. For the record case, you don't even need it, because in practice, you get the correct type inference for free by just spreading one object into another.

[weavejester]

I don't see an asymmetry in the abstraction. Both vectors and maps are associative structures - you can assign a key to a value - the only difference is that vectors have a more constrained keyspace (i.e. ordered, consecutive integers starting from zero).

But that wasn't really my point. Even if we limit `assoc` solely to maps it would still be difficult to type effectively.

For instance, suppose we have some code like:

    (let [m* (assoc m :number 3)]
      (:number m*))

We can see that the return type of this expression is obviously an integer, but what is the type of m*? How do we type m* such that (:number m*) can be inferred to be an integer by the compiler?

Most statically typed languages sidestep this problem: instead of using an open data structure like a map, a closed structure like a record or class is used instead, and these structures must be explicitly typed by the user.

The problem with this approach is that now every record is specific and bespoke. You lose access to all the general-purpose functions that operate on generic data structures, and as records and classes are closed, you also lose the ability to extend them.

This is the ultimate problem with static type systems: you're trading capability for safety. If you're programming within a static type system, there are options that are simply not available or feasible to use.

[IceDane]

> I don't see an asymmetry in the abstraction. Both vectors and maps are associative structures - you can assign a key to a value - the only difference is that vectors have a more constrained keyspace (i.e. ordered, consecutive integers starting from zero).

The asymmetry lies in the fact that it's an overloaded function that's supposed to do the right things every time, but in some cases, it does what is arguably the wrong thing, silently, and in others, it refuses to do the wrong thing and fails loudly. It's better that it fails loudly, of course, but the point is that the ergonomics of the abstraction is lessened because you can't just assume it will work. You effectively have to keep the types of all the things involved in your head and/or trace them to ensure that you don't run into a crash.

> We can see that the return type of this expression is obviously an integer, but what is the type of m? How do we type m such that (:number m*) can be inferred to be an integer by the compiler?

This is trivial in TypeScript. You can see it in action here: https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAtjAvD...

  const m = { name: "weavejester", active: true };

  const mStar = { ...m, number: 3 };
  //    ^? const mStar: { number: number, name: string, active: boolean }

  const x = mStar.number;
  //    ^? const x: number

> This is the ultimate problem with static type systems: you're trading capability for safety. If you're programming within a static type system, there are options that are simply not available or feasible to use.

This is just not true. It's true for some certain specific static type systems, but not true in general, and that brings me back to my original thesis: You just need a sufficiently capable type system with the right properties - structural/row polymorphism, ish, plus type inference. And also my Haskell point: it doesn't have to be an incredibly complicated type system that is beyond mortal ken. TypeScript is already doing this and it's arguably one of the most used programming languages on earth.

[weavejester]

> You effectively have to keep the types of all the things involved in your head and/or trace them to ensure that you don't run into a crash.

You make this sound difficult, but in practice type errors are rare in Clojure and generally caught in the REPL or by tests, since the moment you go down a branch with a type error an exception is thrown.

Contrast this to errors caused via mutable state, which are usually far harder to track down, because the failure condition is more specific.

> This is trivial in TypeScript.

In the example you give you're omitting assoc entirely, which defeats the point. I'm using assoc as a minimal example, but the same principle applies to more complex functions, so replacing assoc with the equivalent expression doesn't tell us whether or not we can effectively type a function that deals with maps.

So lets try doing this properly. At minimum we need something like this:

    type Assoc<M extends object, K extends string, V> =
        Omit<M, K> & Record<K, V>;

    function assoc<M extends object, K extends string, V>(
        m: M, k: K, v: V): Assoc<M, K, V> {
      return { ...m, [k]: v } as Assoc<M, K, V>;
    }

(Note that we need to perform an explicit cast in order to inform TypeScript of the type of the key.)

However, this produces some rather messy types consisting of nested Assocs. In order to get back to something a human can read, we can use an additional Simplify type to force the type system to reduce it back down into an typed object:

    type Simplify<T> = {[K in keyof T]: T[K]} & {};

    type Assoc<M extends object, K extends string, V> =
        Simplify<Omit<M, K> & Record<K, V>>;

    function assoc<M extends object, K extends string, V>(
        m: M, k: K, v: V): Assoc<M, K, V> {
      return { ...m, [k]: v } as Assoc<M, K, V>;
    }

(The empty `& {}` intersection forces normalization, providing a cleaner reported type.)

We're still not done, though, as if we want the same type checking that a class has, we need to ensure that a key cannot be overwritten with a value of a differing type. So we'll type the value argument as well to ensure it matches the type of an existing value within the map:

    type Simplify<T> = {[K in keyof T]: T[K]} & {};

    type Assoc<M extends object, K extends string, V> =
        Simplify<M & Record<K, V>>;

    type AssocValue<M extends object, K extends string, V> =
        K extends keyof M ? (V extends M[K] ? V : never) : V;

    function assoc<M extends object, K extends string, V>(
        m: M, k: K, v: AssocValue<M, K, V>): Assoc<M, K, V> {
      return { ...m, [k]: v } as Assoc<M, K, V>;
    }

So this is possible to type in TypeScript (to its credit), but is it "trivial"? And is this type signature significantly less complex than one might find in Haskell?