[improvement] Generate more lenient enums

[p-szm]

Fixes #80

Based on #85 (which ended up being reverted) but I used namespaces instead of objects in order to preserve back compatibility.

Before this PR

We generated TypeScript enums which were difficult to use across library boundaries since equivalent enums could no be assigned to each other. See #80 for more details

After this PR

We generate a union type and a namespace.

Notes

I tested the edge case in #85 (comment) which caused the original attempt to be reverted and this time everything compiles correctly:

https://www.typescriptlang.org/play?#code/PTAEHkBsBNQYwPbQKYHNkDsCwAoZAPABwQCcAXeBDAZwswFcBbCAGQBEB9AUQDkBVALKgA3rlDjQAMXDhQAXlAAiaeEUAaMRIBCAQQBK8pbr3rN43QC1Diy6ZwBfXLhCgeyAO6UU6bHiKkKDABDRmRqQiC4ZFcuAHVufiFRHAlQAmJyUDIAT0JolWsVRQBuMzT-TJy80GNrYxKy9ICs3OjLOp0LBpxGiopEGgoChWUZbtSmzIHaGv0Ok1KUiUn+qhn2kdtFxz8MiironjiEwUMAa2RshAAzFrybmPjeQUXnMChYACNkAAsggDcAJYIegkXDXegYOBkYEYUDUeifajIMgcBAwAAUERIIQAXKxOM8BAA6AoAHwJJxJlgAlCIdhCoTCqKAUNcgvRIGQAApBHGMNGY-5BSD0aIKcDsKmkmR04Q7Nkcrm8-mC6AYyWExLE4w0144FxuTzfP5AkEkUAAWkojEIgMgYXBkOhsPhiORqIwHixfLxj2l5P9RJ1nTloB2bykQXt1CyCBtdodoEBGGFkEBsARSJRTqZrsY2UkzoAzD7+fiuPgyJhoNQADxHJ6JNRKa4IBCKAB8cojOCAA

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Generate more lenient enums.

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[ferozco]

Lets try out the code change in a large internal repo and then we should be good to go

[p-szm]

I tested this on one of our conjure generated packages that has a bunch of enums, and it all seems to work.

[ferozco]

Looks good to me

[tjwilson90]

This change breaks the generated code for empty enums. Before this change

types:
  definitions:
    default-package: "com.palantir.cosmos.generated"
    objects:
      AircraftGeoPointField:
        values: []

generated

export enum AircraftGeoPointField {
}

and after this change it generates

export namespace AircraftGeoPointField {
}

export type AircraftGeoPointField = keyof typeof AircraftGeoPointField;

which fails to compile with the error

> Task :cosmos-api:compileTypeScript FAILED
cosmos-generated/aircraftGeoPointField.ts(4,50): error TS2708: Cannot use namespace 'AircraftGeoPointField' as a value.
cosmos-generated/aircraftGeoPointField.ts(4,50): error TS4081: Exported type alias 'AircraftGeoPointField' has or is using private name 'AircraftGeoPointField'.

[p-szm]

thanks for spotting @tjwilson90 . I think we can generate this instead for empty enums:

export namespace AircraftGeoPointField {
}

export type AircraftGeoPointField = never;

I can put up a fix first thing tomorrow morning if you think that works, but I'm also happy to revert in the meantime.

[p-szm]

Also, I'm curious - do you actually use empty enums in your typescript code, or was this an API in progress?

[ferozco]

@patrickszmucer is never the right type here, seems like never can be assigned to anything?

[tjwilson90]

My conjure api has a lot of redundancy / self similarity and is itself generated (you can imagine Aircraft being one of many different types of objects). I generate a similar search api for all of them. Some of those generated types (like aircraft) don't have any geo point fields, so the corresponding enum is empty.

[tjwilson90]

I don't mind waiting until tomorrow. It blocks an autobump which isn't high priority.

[p-szm]

@ferozco you're right - seems like never is assignable to anything so it would be a bit dangerous to generate it (although I'm not too worried because empty enums should be super rare).

However, I think I found a better solution - we can use void instead which can't be instantiated and also can't be assigned to anything else. Typescript folks did something similar to the window.name variable which used to be never but it was changed to void for better type safety - microsoft/TypeScript-DOM-lib-generator#883

[p-szm]

Fix here #176