Decorators Status Update
Chris Garrett
2020-06
Meeting Updates
- Bi-weekly, Tuesdays at 10am PST (5pm UTC)
- Next meeting is June 16th
- Agenda doc in Reflector. Anyone can add items before the next meeting and join.
- Open to TC39 members and invited guests
Design Space Updates
Decorators must be static
There is a one-time cost that is paid during parsing or the initial compile.
- Decorators should have no per-class literal evaluation cost by design.
- Engines should be able to generate code for decorators during the initial compile. That is, modules or class bodies should not need to be reparsed/recompiled.
- Definition Static
- Application Static
- Built-Target Static
1. Definition Static
Static through analyzable definition
// tracked.mjs
export decorator @tracked {
@initialize((instance, name, value) => {
instance[`__internal_${name}`] = value;
})
@register((target, name) => {
Object.defineProperty(target, name, {
get() { return this[`__internal_${name}`]; },
set() { this[`__internal_${name}`] = value; this.render(); },
configurable: true
});
})
}Definition Static Design Space
- Definitions must be statically analyzable
- No arbitrary expressions
- Definitions must decompose into a number of known operations
- Definitions must be parsed before they are applied
2. Application Static
Static through exactly one meaning applying a decorator
function logged(enabled) {
return () => {
get(target, instance, prop, value) {
return value
},
set(target, instance, prop, value) {
if (enabled)
console.log(prop, value)
return value
}
}
}Application Static Design Space
- Decorators must have exactly one meaning
- That meaning must boil down to a transform that is known ahead-of-time
- Every decorated value can have a different meaning
- Classes
- Class Fields
- Class Accessors
- Class Methods
3. Build-Target Static
Static through desugaring decorators to a static output via build-tools
Build-Target Static Design Space
- Decorator syntax still needs to be usable without transpilation/desugaring. Could be dynamic, as long as common use cases are compiled out by build tools.
- Decorators may have many meanings
- Build tools may need to be able to agree on definitions of decorators ahead of time, or come up with some kind of community standard
- Output-size is a constraint - adding too many bytes could be an issue
- So far we've added a variety of concerns and constraints
- Unclear if there is any way to solve every constraint as is
Ecosystem Audit
- Examining top decorators libraries in the ecosystem today
- Categorizing decorators by capabilities and behaviors
- Goal is to gather data about most common use cases to help inform the design process
Decorators Status Update 2020-06
By pzuraq
Decorators Status Update 2020-06
- 1,506
