61 replies

Typescript generic field selection assumes type incorrectly

I was working on a generic lib-like api for something I'm working on, and got stumped by unexpectedly wrong typescript generic field selection.

E.g. if

<Foo extends Something>(args: Foo)

<Foo extends Something>(args: Foo)

, then you'd think

args.field

args.field

is typed as

Foo['field']

Foo['field']

, but it's not, it's simplified down instead.

Here's a minimal repro:
https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABFApgZygHgCqJQD1TABM1EBvRAQwC4LFg446MAnGMAc0QF9eA+ABRVWnOtgCUFAFCJErFFBCskIzgDoqAbmk9p0iAgyJiVKFUQBeZOiiDKteo2aIARK4A0iAEYi673l4JHUMwNDgAGxR1CLhOQVNzdV9WYOkgA

Is there known ways of getting around this without using

as

as

? It feels so wrong. E.g. here's what i have to do to fix it in my real code:

z.object({
  functionName: z.literal(args.name as ToolArgs["name"]),
  args: args.data.args as ToolArgs["data"]["args"],
  responseData: makeToolResponseDataSchema(
    args.data.internalResponseData as ToolArgs["data"]["internalResponseData"],
    args.data.externalResponseData as ToolArgs["data"]["externalResponseData"]
  ),
})

z.object({
  functionName: z.literal(args.name as ToolArgs["name"]),
  args: args.data.args as ToolArgs["data"]["args"],
  responseData: makeToolResponseDataSchema(
    args.data.internalResponseData as ToolArgs["data"]["internalResponseData"],
    args.data.externalResponseData as ToolArgs["data"]["externalResponseData"]
  ),
})

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TTC

Theo's Typesafe Cult•2y ago•

61 replies

Arduano

Typescript generic field selection assumes type incorrectly

I was working on a generic lib-like api for something I'm working on, and got stumped by unexpectedly wrong typescript generic field selection.

E.g. if

<Foo extends Something>(args: Foo)

<Foo extends Something>(args: Foo)

, then you'd think

args.field

args.field

is typed as

Foo['field']

Foo['field']

as

as

? It feels so wrong. E.g. here's what i have to do to fix it in my real code:

z.object({
  functionName: z.literal(args.name as ToolArgs["name"]),
  args: args.data.args as ToolArgs["data"]["args"],
  responseData: makeToolResponseDataSchema(
    args.data.internalResponseData as ToolArgs["data"]["internalResponseData"],
    args.data.externalResponseData as ToolArgs["data"]["externalResponseData"]
  ),
})

z.object({
  functionName: z.literal(args.name as ToolArgs["name"]),
  args: args.data.args as ToolArgs["data"]["args"],
  responseData: makeToolResponseDataSchema(
    args.data.internalResponseData as ToolArgs["data"]["internalResponseData"],
    args.data.externalResponseData as ToolArgs["data"]["externalResponseData"]
  ),
})

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The Playground lets you write TypeScript or JavaScript online in a safe and sharable way.

Typescript generic field selection assumes type incorrectly

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