// SPDX-License-Identifier: CC-BY-SA-4.0
= AffineScript: Design Vision
Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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== What AffineScript Is

AffineScript is a *web programming language*. Its purpose is to replace the
fragmented x-script ecosystem — JavaScript, TypeScript, ReScript, CoffeeScript,
ActionScript, and their successors — with a single language that solves the
problems they collectively fail to solve, without inheriting their compromises.

It compiles to *typed WebAssembly*. Not JavaScript. Not JavaScript-with-types.
WebAssembly, where the type guarantees survive into the runtime.

== The Core Identity

AffineScript is *affine-first*. This is its permanent resting state, not a
stepping stone toward something stricter. A resource can be used at most once
— that is the natural condition of the language. Unrestricted use is the
explicit opt-in. Linear (exactly once, must be used) is a stricter annotation
available when needed.

This is different from Ephapax, where affine is the enabler of linear.
In AffineScript, affine IS the goal. The language rests here.

== What Is Wrong With X-Scripts

The entire x-script world has six unsolved problems:

[cols="1,3", options="header"]
|===
| Problem | What it looks like

| *Null is a lie*
| `string \| null \| undefined` — two different nothings. `?.` and `??` are
  band-aids that confirm the wound. `strictNullChecks` is opt-in.

| *Mutation is invisible*
| Everything is mutable by default. You pass an object into a function and it
  may come back changed, or something may hold a reference and change it later.
  No type system has a model for this.

| *Errors throw sideways*
| `throw` accepts anything. `catch (e: unknown)` means you know nothing.
  Every function that can fail either lies in its type signature or forces
  untyped exception handling.

| *Async is bolted on in layers*
| Callbacks → Promises → async/await → hooks → signals. Each layer was added
  because the previous one was insufficient. None composes. `async` infects
  everything it touches.

| *Complex types become unreadable*
| TypeScript generics with `infer`, conditional types, and mapped types produce
  code nobody can read six months later.

| *The runtime is still JavaScript*
| Types are erased. Runtime errors that should be compile errors still happen.
  The type system floats above the execution, not in it.
|===

== How AffineScript Answers Each

[cols="1,2,2", options="header"]
|===
| Problem | X-Script answer | AffineScript answer

| Null
| `T \| null \| undefined`, `?.`, `??`
| `Option[T]`. No null. No undefined. No two different nothings.

| Mutation
| Nothing. `const` does not help. `readonly` is partial.
| `own`/`ref`/`mut` as surface syntax for quantity annotations. Ownership
  is in the type. You know from the signature what a function does.

| Errors
| Untyped `throw`/`catch`
| `Result[T, E]`. Errors are in the return type. Nothing is hidden.

| Async/effects
| async/await + hooks + observables (incompatible)
| Algebraic effects. One model for all of: DOM, fetch, localStorage, timers,
  websockets, state. Composable by design.

| Complex types
| `infer`, conditional types, mapped types
| Row polymorphism. `{ name: String, ..r }` accepts any record with a name
  field. Structural, readable, compositional.

| Runtime
| Types erased at runtime
| Typed WASM. Types survive into the runtime. The guarantees are real.
|===

== The Type System

=== Affine-First

Resources are affine by default: used at most once. This prevents the entire
class of bugs where a reference is aliased and mutated through multiple paths.

[source]
----
own T    — caller transfers ownership (affine: must not use after)
ref T    — borrowed view, read-only (unrestricted: can use many times)
mut T    — exclusive mutable access (affine: one writer, enforced)
----

These are surface syntax for the underlying quantity annotation system (QTT).
There is no separate borrow checker. The affine type system handles it.

=== Structural / Row-Polymorphic

Records are structural, not nominal. A function that operates on
`{ x: Int, ..r }` accepts any record with an `x` field. Type aliases give
names to shapes — they do not create new nominal types.

[source]
----
// Accepts any record that has a name field — nothing else required
fn get_name[..r](obj: { name: String, ..r }) -> String = obj.name

// Type alias — a name for a shape, not a new nominal type
type point = { x: Int, y: Int }
----

This fits the web naturally. JSON is structural. APIs return open shapes.
Row polymorphism models web data correctly without the complexity of
TypeScript's mapped and conditional types.

=== Algebraic Effects

The web is effects. Every interaction with the browser — reading the DOM,
making a network call, writing to storage, receiving a timer event — is an
effect. AffineScript gives all of these a single, composable model.

[source]
----
effect Http {
  fn get(url: String) -> Response;
  fn post(url: String, body: String) -> Response;
}

effect Async {
  fn await[T](p: Promise[T]) -> T;
}

// Effects are in the type signature. Compose freely.
fn fetch_user(id: Int) -> User / Http, Async {
  let resp = Http.get("/users/" ++ show(id));
  Async.await(resp.json())
}
----

The handler — installed at the boundary of your application — is the affine
resource. Individual invocations are capability uses. No `async` keyword
contamination. No incompatible effect libraries. One model.

=== No Null, Typed Errors

[source]
----
// Nothing is hidden
fn divide(a: Int, b: Int) -> Result[Int, DivisionByZero] {
  if b == 0 { Err(DivisionByZero) } else { Ok(a / b) }
}

// No null, no undefined
fn find_user(id: Int) -> Option[User]
----

== Ergonomics: The Real Ambition

AffineScript's type system is not its primary pitch. The pitch is:
*this should be easier to write than JavaScript, Python, or pseudocode.*

That is the hard problem. The goal is that a JavaScript developer, a Python
developer, or someone who has never programmed before can engage with this
language without first needing to understand type theory.

The rules of the type system should be invisible when you are writing
straightforward code. You should feel them only at the edges — when you try to
use a resource twice, when you try to pass `null` where a `String` is expected,
when you try to call a function that can fail without handling the failure.

The type system enforces what good programmers already do instinctively.
AffineScript just makes it impossible to forget.

=== What Familiar Looks Like

[cols="1,1,1", options="header"]
|===
| Pseudocode / Python | JavaScript | AffineScript

| `x = 5`
| `const x = 5`
| `let x = 5`

| `def greet(name):`
| `function greet(name) {`
| `fn greet(name: String) -> String {`

| `if x > 0: return "yes"`
| `if (x > 0) return "yes";`
| `if x > 0 { "yes" } else { "no" }`

| `result = maybe_thing or default`
| `result ?? default`
| `result.unwrap_or(default)`

| `try: ... except: ...`
| `try { ... } catch(e) { ... }`
| `match divide(a, b) { Ok(v) -> v, Err(_) -> 0 }`
|===

The goal: the AffineScript column should feel like the obvious way to write
the thing, not like a translation from a foreign language.

== Compilation Target

Typed WebAssembly (WasmGC). Not JavaScript. The compiler produces WASM
modules where the type information survives into the runtime. This is the
right target for the next generation of web applications, browser games,
and edge compute.

A thin JavaScript interop layer exists for calling Web APIs and for incremental
adoption from TypeScript codebases, but it is interop — not the target.

== What AffineScript Is NOT

* Not a JavaScript variant. Not "TypeScript with better types." A clean break.
* Not Rust for the web. Rust's borrow checker is a solution to a problem
  AffineScript does not have (because it has affine types instead).
* Not a functional-programming language that happens to have a WASM backend.
  It is a web language that happens to have a sound type system.
* Not Ephapax. Ephapax is affine-as-enabler-of-linear. AffineScript is
  affine-as-home. Different purpose, different audience, different character.

== Relationship to Ephapax

Independent. Neither language constrains the other's design.

They converge at typed WebAssembly. An AffineScript module and an Ephapax
module compiled to typed WASM can interoperate at the binary level — because
they speak the same bytecode, with agreed type layout conventions.

The aggregate library lives at that convergence point: shared WASM-level
infrastructure (ABI conventions, stdlib types in typed WASM, linking
conventions) that neither language owns but both can consume.

See `docs/specs/AGGREGATE-LIBRARY.adoc` and the `typed-wasm` repository for
the convergence infrastructure.
