Add enum support

[alecgibson]

Fixes #67
Fixes #216

The enum is a popular aspect of TypeScript, and can be embedded in
interfaces as a concise, but descriptive, shorthand for literal string
unions:

enum Colour {
  White = '000000',
  Black = 'ffffff'
}

This change adds an exported member enum to io-ts, based on
this suggestion by @noe132

It means that enums can be reused directly in io-ts:

const T = t.enum(Colour)

[alecgibson]

One place this does seem to fall down is that you lose the casting when extracting an interface. I can't quite figure out how to do that (possibly something to do with the encode function?). For example:

enum Colour {
  White = '000000'
}

const Style = t.type({
  colour: t.enum(Colour)
})

interface IStyle extends t.TypeOf<typeof Style> {}

const style: IStyle = {
  colour: Colour.White
}

const colour = style.colour // colour has type "any" rather than "Colour"

I tried tweaking the signature for enumType:

const enumType = <E, A = E[keyof E]>(e: E, name: string = 'Enum'): EnumType<A> => {
  const is = (u: unknown): u is A => Object.keys(e).some((k) => e[k as keyof E] === u)
  return new EnumType<A>(name, is, (u, c) => (is(u) ? success(u) : failure(u, c)), identity)
}

...which seems to only succeed in disallowing a decode to string (which I guess is correct?), but still doesn't correctly cast.

[jessmorecroft]

This looks like it might work better, for string enums at least...

const enumType = <E>(
  e: { [key: string]: E },
  name: string = 'Enum',
): EnumType<E> => {
  const is = (u: unknown): u is E => Object.keys(e).some(k => e[k] === u);
  return new EnumType<E>(
    name,
    is,
    (u, c) => (is(u) ? t.success(u) : t.failure(u, c)),
    t.identity,
  );
};

[saa18877433]

Re: [gcanti/io-ts] Add `enum` support (#366) 2020-02-16 2:56 غرينتش-08:00, jessmorecroft <notifications@github.com>:

…

This looks like it might work better, for string enums at least... ``` const enumType = <E>( e: { [key: string]: E }, name: string = 'Enum', ): EnumType<E> => { const is = (u: unknown): u is E => Object.keys(e).some(k => e[k] === u); return new EnumType<E>( name, is, (u, c) => (is(u) ? t.success(u) : t.failure(u, c)), t.identity, ); }; ``` -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: #366 (comment)

[noe132]

Hey, since number enums have remapping,
https://www.typescriptlang.org/docs/handbook/enums.html#reverse-mappings
I think it's essential to check if the key is the remapped one.

enum E {
  A = 1
}

// compiles to
const E {
  A: 1,
  '1': 'A',
}

In this case, 1 is valid, but 'A' is not.
https://www.typescriptlang.org/play?#code/KYOwrgtgBAolDeAoKUCCUC8UCMiC+QA

I've updated my code here #216 (comment)

[abrkn]

Any update on this?

[aroman]

+1 this would be a great addition, as currently I need to manually copy/paste my enum values into a t.union of t.literals

[alecgibson]

Okay I've updated this, and it now correctly casts!

Also updated to include @noe132 's update for checking for reverse mapping

[Andrapyre]

Any update on this from @gcanti ?

[halbich]

Hi, what is the current status of this feature? We'd love to have enum support :)

[taylor-shift]

@gcanti How do you feel about merging this in?

[martindines]

Is there anything stopping this feature from being merged?