Function infer generic parameter key from input on another key

[cunning mortar]

Here's an example, I would like to make the expect-error directives pass. I recall seeing a solution to this in the past, but can't remember now.

interface Input<T extends 'a' | 'b' = 'a' | 'b'> {
  t: T
  u: T extends 'a' ? (1|2) : (3|4)
}

const create = <$Input extends Input>(x: $Input) => x

// create({ t: 'a', u: /* should be 1|2 <- struggling to get this working */ })

create({t:'a',u:1})
create({t:'a',u:2})

// @ts-expect-error
create({t:'a',u:3})

// @ts-expect-error
create({t:'a',u:4})

[thorny dew]

You want a DU.

!hb discriminated union

[random path]

Get Started / TypeScript for Functional Programmers

Discriminated Unions

[cunning mortar]

Taking a look

I'm aware of DU. That doesn't fit my use-case. The relationship between 't' and 'u' is actually more complex than you see here.

My question is how to infer u from t in turn from the value passed to create

Answer here I think https://stackoverflow.com/questions/78361339/typescript-function-infer-generic-parameter-key-from-input-on-another-key/78361428#78361428

[thorny dew]

Show the more complex relationship then, I can only answer based on what you show.

[cunning mortar]

shifting the generic of create seems to be the thing to do for this.

[thorny dew]

const create = <T extends 'a' | 'b'>(x: Input<T>) => x

Would be an easy fix.

[cunning mortar]

But there's a problem with that, which is, I need the Input<T> available on the result type too

[thorny dew]

Well, I just wouldn't write Input<T> like that and use a DU.

[cunning mortar]

I'll give you the more complex example, sec.

[random path]

jasonkuhrt#0

Playground Link

Preview:```ts
//declare global {
interface Foo {
A: {
hasThing: true
}
}
//}
//declare global {
interface Foo {
B: {
hasThing: false
}
}
//}

type Name = keyof Foo

interface Input<$Name extends Name = Name> {
t: $Name
u: Foo[$Name] extends true ? (1|2) : (3|4)
...```

You can choose specific lines to embed by selecting them before copying the link.

[cunning mortar]

Revised: https://www.typescriptlang.org/play?#code/PTAmFMGMBsEMCdwAIDm0D2AjW0kG8AoJJASwDsAXceAM1kmQDF118jikBBALjY44AWsAM4AVAeRS8K8AK7h2xAL7sVINWChxEqDNlx5i7clVr0mLPhwBCvQv2JCxEslKR1owhfxXKC6ggIKAE8AB2QAOVgAW2QAXiQAa3Bg9BokZnRAk2o6BiQASTJQ2QoAHgASKNikcAAPKjJQYSRq+NaY8AA+KwpeKs72WV5MgG0B2IBdWobwJpaZeSQAfiQACgBGAB8AJgBKJF41gGYtgBY9ghV-YCQAYQB5CIBlUQAlAFU70Qe3pEDIOgyMIKEhIIhYFQkAlKkUSqD6o1moViqUums6v04aU9ljURRRgByWSE6ZxHqGRAUWTwMhIWRNcA0cjgUBIETssjBJDXEBIUQAUVezxuSAAtBLJVLpTLZXL5WLAtBwKC6tCwRCqGs8H1CZxCQAaencDZKS5qhIbJB8spipDCAToWTQNkABU4zxFfIAAhRhGL6uFIBQA-B4Oh4AQLUgdoE+QqE4mkwCgSCkNyEuDwJDwNrdfqjcMdmbRb7-YGoCHqOHI9GNlH1TtrbdbfbHc63R6RaKk72+1K47cywG6kGq2GIwQszm89w9YbjccSz3+6vE4OkMOK8HQzWp5rczq5wXjWczUA

[random path]

@cunning mortar Here's a shortened URL of your playground link! You can remove the full link from your message.

jasonkuhrt#0

Playground Link

Preview:```ts
//declare global {
interface Foo {
A: {
hasThing: true
}
}
//}
//declare global {
interface Foo {
B: {
hasThing: false
}
}
//}

type Name = keyof Foo

interface Input<$Name extends Name = Name> {
t: $Name
u: Foo[$Name] extends true ? (1|2) : (3|4)
...```

You can choose specific lines to embed by selecting them before copying the link.

[thorny dew]

Are you in control of Foo?

Actually, that doesn't really matter.

As a side note it seems like your code is wrong, I assume what you really want is Foo[$Name]['hasThing'] extends true not Foo[$Name].

[cunning mortar]

@thorny dew Thanks yes, fixed!

https://www.typescriptlang.org/play?#code/PTAmFMGMBsEMCdwAIDm0D2AjW0kG8AoJJASwDsAXceAM1kmQDF118jikBBALjY44AWsAM4AVAeRS8K8AK7h2xAL7sVINWChxEqDNlx5i7clVr0mLPhwBCvQv2JCxEslKR1owhfxXKC6ggIKAE8AB2QAOVgAW2QAXiQAa3Bg9BokZnRAk2o6BiQASTJQ2QoAHgASKNikcAAPKjJQYSRq+NaY8AA+KwpeKs72WV5MgG0B2IBdUYByJ3FJGcnahvAmlpl5JAB+JAAKAEYAHwAmAEokXj2AZiOAFjOCFX9gJABhAHkIgGVRACUAKpvUQfP5IQKQdBkYQUJCQRCwKhIBKVNorRrNDqxAA0SAqRRKsPqGJaBNKqM6XS6ezq-TJFDOdOKpVmsiWyJ6hkQFFk8DISFkTXANHI4FASBEErIwSQzxASFEAFFft8XkgALSarXanW6vX6g3qwLQcCwurIuEIqh7PB9GacGa44YHJSPc0JA5IeVldVIYQCdCyaDigAKnG+qvlAAEKMJ1fVwpAKPH4PB0PACO6kCdjaakHUThb4eBEeAbXbrI6Bdxrq61TG4wmoMnqGmMwWLXdc2brkWrWXbdwZpWndw7nXo7H43VEy3U+nM72EtdAvLDeuN5uIVCYUgZQli6Xy0OHaOThPXg3p7OU23MxaDveEoXvb7-YHg0gwxHV69N-+AO1X8kCvJsk1vBdD2tQd7SrYZa0eNVAOQrd6ynMC5zvKCBztU9q3HM4gA

[random path]

nonspicyburrito#0

Playground Link

Preview:```ts
interface Foo {
A: {
hasThing: true
}
B: {
hasThing: false
}
}

type Input = {
[K in keyof Foo]: {
t: K
u: Foo[K]["hasThing"] extends true ? 1 | 2 : 3 | 4
}
}[keyof Foo]

// CONSTRUCTOR

const create = <T extends Input>(x: T): T["u"] =>
...```

You can choose specific lines to embed by selecting them before copying the link.

[cunning mortar]

Damn, thanks @thorny dew ! This is interesting...

[thorny dew]

Input is a DU, it's just generated from Foo.

You can hover over Input to see the generated output.

[cunning mortar]

Was about to say, you're back to DU but in a more dynamic/computed way

Yeah, and the autocomplete when using create works great too...