Unable to assign function to type with generic parameter

Question

I have a problem which boils down to trying to assign a function with a specific parameter type to a variable expecting a function with a generic type:

const s: <T>(v: T)=>string 
  = (v: string[])=>(v[0]||'x');

Playground version.

Typescript gives the following error:

Type '(v: string[]) => string' is not assignable to type '(v: T) => string'. Types of parameters 'v' and 'v' are incompatible. Type 'T' is not assignable to type 'string[]'.

This error doesn't make much sense to me, as it seems like string[] is a perfectly reasonable type to use for the generic T.

There is a related question that has the same underlying problem, but the answer there was specific to circumstances in that question: Why is this TypeScript class implementing the interface not assignable to a generic constraint extending the interface?

<T>(v: T) => string means the type of a function which can consume anything and output a string; your function can only consume an array of strings. — kaya3
– kaya3, Commented Jan 14, 2021 at 5:30
const s: <T extends string[]>(v: T) => string = (v: string[]) => (v[0] || 'x'); — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Jan 14, 2021 at 7:19

Nishant · Accepted Answer · 2021-01-14 05:41:24Z

1

As mentioned by kaya3, you are trying to assign a function that take a specific type, string[], to a function that takes generic types . What you may do instead, is, define a type with the generic signature. And then create a function that takes is one specific instance of that generic signature. Something like this:

type MyType<T> = (v: T) => string;
const s: MyType<string[]>  = (v: string[]) => v[0] || 'x' ;

If you have just one type, this would be sufficient:

const s  = (v: string[]) => v[0] || 'x' ;

edited Jan 14, 2021 at 5:41

answered Jan 14, 2021 at 5:39

Nishant

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1 Comment

vipcxj Over a year ago

That was my answer too, but I was six seconds late

Chris Yungmann · Accepted Answer · 2021-01-14 05:30:38Z

0

You are trying to take a function that takes an array of strings and returns a string, and assign that function to a variable that takes any T and returns a string. A function that takes an array of strings as input cannot take any T as input.

answered Jan 14, 2021 at 5:30

Chris Yungmann

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