CODEWITHSUNDEEP
typescript
Trident D'Gao
Trident D'Gao

Reputation: 19762

Cannot overload a generic function in TypeScript

What am I doing wrong here:

export function fail<a>(problem: SomeProblem): a;
export function fail<a>(message: string): a;
export function fail<a>(messageOrProblem: any): a { throw Error(); }

the compiler says:

TS2148: Build: Overload signature is not compatible with function definition.

Upvotes: 1

Views: 2182

Answers (1)

Ryan Cavanaugh
Ryan Cavanaugh

Reputation: 221392

The type parameters here are considered 'different' because they come from different places. You should write this like so:

export function fail<a>(problem: SomeProblem): a;
export function fail<a>(message: string): a;
export function fail(messageOrProblem: any): any { throw Error(); }

As an aside, using a generic type argument only in a return value position is sort of an anti-pattern. Since you have no way to determine what value to return based on a, it's much more accurate to return any than to return an uninferable generic type. I call this the "moving the cast" pattern:

// Bad
x = fail<string>('foo'); // This looks more typesafe than it is
// Good
x = <string>fail('foo'); // Let's be honest with ourselves

Upvotes: 6

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