Can you provide an explanation for the error in bar1 but not in bar2? What are the fundamental differences between them?
const foo1 = <E extends (a: unknown)=>void>(e:E)=>console.log(e)
const bar1 = <Arg extends unknown>()=>foo1((a:Arg)=>console.log(a))
const foo2 = <Arg extends unknown>(e:(a: Arg)=>void)=>console.log(e)
const bar2 = <Arg extends unknown>()=>foo2((a:Arg)=>console.log(a))
foo1.E.a is expected to encompass or be equal to unknown, however, bar1.Args is a subset or equal to unknown
foo2.Args should be a subset or at the same level as unknown, while bar2.Args is also contained within or equal to unknown.
Understanding Variance, Covariance, Contravariance, and Bivariance in TypeScript
The solution provided by @lei li is a good starting point, but for a more precise approach, changing 'a' to never makes the code more versatile and "extendable".
const foo2 = <E extends (a: never)=>void>(e:E)=>console.log(e)
const bar2 = <Arg extends unknown>()=>foo2<(a: Arg)=>void>((a)=>console.log(a))
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