Repository navigation
callbacks should select the right overload #35501
Description
Activity
Another example, where both overloads would match
declare function fn(x: string): string; declare function fn(x: string): number; declare function map<A, R>(fn: (item: A) => R, list: A[]): R[]; const mapped = map(fn, ['1'])
mappedshould be of typenumber[] | string[]Reacted by Caleb JasikHere's my current workaround
declare function fn(x: string): string; declare function fn(x: string[]): string; declare function map<A, R>(fn: (item: A) => R, list: A[]): R[]; const mapped = map((x) => fn(x), ['1'])
Wrapping
fndoes get TS to choose the right overloadGeneric inference is not able to do overload resolution, though your examples all don't need overloads in the first place.
The examples are silly yes, but they do show how overloads could be improved. I get these scenarios quite often (on ramda especially). So for now, I wrap with
() =>which does the trick, so I thought this could have been solved easily.Here is the simplest possible replication of this I could come up with: [Playground]
interface Callback { (value: number): any (value: string): any } function fn(callback: Callback) { // ... } fn(value => { // ^ Parameter 'value' implicitly has an 'any' type. })
Here is a real world example: [Playground]
interface Callback<T> { (error: null, result: T): unknown (error: Error, result: null): unknown } interface Task<T> { (callback: Callback<T>): unknown } export function series<T>(tasks: Task<T>[], callback: Callback<T[]>): void { let index = 0 let results: T[] = [] function next() { let task = tasks[index] if (!task) { callback(null, results) } else { task((error, result) => { // ^ Parameter 'error' implicitly has an 'any' type. // ^ Parameter 'results' implicitly has an 'any' type. if (error) { callback(error, null) } else { results.push(result) next() } }) } } next() } series([ cb => setTimeout(() => cb(null, 1), 300), cb => setTimeout(() => cb(null, 2), 200), cb => setTimeout(() => cb(null, 3), 100), ], (error, results) => { // ^ Parameter 'error' implicitly has an 'any' type. // ^ Parameter 'results' implicitly has an 'any' type. if (error) { console.error(error) } else { console.log(results) } })
Just to be clear: Note that even though #42620 is listed above as closing this issue, Wesley Wigham (@weswigham) says it only fixed the comment before this one, not the whole issue. So this issue is still a design limitation and is not fixed.
TypeScript Version: 3.8.0-dev.20191121
Search Terms:
callback overload mismatch
Code
Expected behavior:
TypeScript does infer
Aasstringand could choose the right overloadfn(x: string), decline the ones that don't fit.Actual behavior:
Checks the last overload only, so it shows an error.
Playground Link: http://www.typescriptlang.org/play/?ssl=7&ssc=1&pln=1&pc=1#code/CYUwxgNghgTiAEAzArgOzAFwJYHtVNQAoAPALngGcMYtUBzASnKpvoG4AoUSWBFdbHgIlm1WnQDaAXSaUx7Dl3DQ4SNJlz4AtlAAOAHgCCAGngAlAHyFEqcoSwYQW8oYbwAvBfOmIWKi+lZM2lODjA8KngdXV0QYA8ovWtUUwkAcgBGNJkOIA
Related Issues: