Const assertions

[goodmind]

Proposal

https://devblogs.microsoft.com/typescript/announcing-typescript-3-4/#const-assertions

const foo = ([1, 2, 3]: const) // Type is [+1, +2, +3] (how to do read-only elements on tuple?)
const bar = ('foo': const) // Type is 'foo'
const baz = ({foo: 1}: const) // Type is '{| +foo: 1 |}'

Use case

More explicit version of #7607
Also would eliminate need for Object.freeze as tool for casting to singleton strings
Object.freeze is also a bit broken with regards to subtyping #7964

Not sure how sound it is, given that it is type cast essentially

Alternative

const foo = Object.freeze('foo') // Type is 'foo'
const baz = Object.freeze(1) // Type is 1
const bar = Object.freeze([1, 2, 3]) // Type is [+1, +2, +3] (how to do read-only elements on tuple?)

[kevinbarabash]

This would be really handy for objects as well. Some of our redux code at work defines constants objects like so:

const Constants = {
   Foo: ("Foo": "Foo"),
   Bar: ("Bar": "Bar"),
   ...
};

except the constants are much longer. It would be nice to be able to do:

const Constants = ({
   Foo: "Foo",
   Bar: "Bar",
   ...,
}: const);

[goodmind]

Thia is currently possible with Object.freeze, but only $Values supports it

[kevinbarabash]

Does this mean $Values would return a tuple of type ["Foo", "Bar", ...]?

[goodmind]

@kevinbarabash, no, union

[Eliav2]

something new here? this static type checker really has some potential but these missing typescript equivalent features prevent many of us from trying flow seriously...

[SamChou19815]

as const support is now enabled by default