Is there support for template literal types?

[David-van-der-Sluijs]

In Typescript there is a thing called Template Literal Types. So I've made this example type:

type Owner = 'Bob' | 'Jeff';
type Dog = `${Owner}'s dog`;

The resulting union type of Dog now is "Bob's dog" | "Jeff's dog".

Is there a way to create a corresponding runtype for this?

[yuhr]

Not an intuitive one, but you can achieve it by using Constraint runtype.

const Owner = Union(Literal('Bob'), Literal('Jeff'))
const Dog = String.withConstraint(
  s =>
    Owner.guard(s.match(/^(?<owner>.+)'s dog$/)?.groups?.owner) ||
    'invalid owner',
)

Dog.check("Bob's dog")
// OK

Dog.check("Alice's dog")
// ValidationError: Failed constraint check for string: invalid owner

I think it's better to add this as a separate runtype if possible (especially for the sake of correspondence to TS's type system), but I'm not sure how we should implement. Maybe we could use tagged template?

[yuhr]

Hm, I thought it's concise to be able to write it like this:

import { Union, Literal, Template } from 'runtypes'
const Owner = Union(Literal('Bob'), Literal('Jeff'))
const Dog = Template`${Owner}'s dog`

But then Static<typeof Dog> is not possible currently as TS doesn't populate TemplateStringsArray (the type of first parameter of the tagged template function) as a tuple of string literal types. My ideal behavior is suggested here, but it's not likely to be available at least in the near future.

That said, I confirmed runtime type checking is possible in the above syntax. I'm going to file a PR.