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Change the Either type signature

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This commit is contained in:
Michal Vanko 2022-03-16 15:30:08 +01:00
parent 8abf4923a4
commit 5f84339d77
1 changed files with 19 additions and 13 deletions
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32
_posts/blog/2022-02-28-error-handling-with-either-type.md
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@ -7,6 +7,7 @@ tags:
- Development
- Guide
---
We have started a new small internal project for automating a few workflows around counting worked hours and time offs.
## Application architecture
@ -24,7 +25,7 @@ The goal is to handle errors that are expected. Unexpected errors should still b
## Error types
Let's create 2 types of `Error`s that all the other `Error`s can be extended from.
These will allow us to distinguish if the thrown `Error` should be presented to the user. We want to handle `InternalError`s differently. We might log them to different logs or trigger alarms before we convert them to a different `PublishableError`.
These will allow us to distinguish if the thrown `Error` should be presented to the user. We want to handle `InternalError`s differently. We might log them to different logs or trigger alarms before we convert them to a different `PublishableError`.
```typescript
export class PublishableError extends Error {
@ -43,10 +44,10 @@ export class InternalError extends Error {
}
}
```
Then, we can create multiple app-specific errors by extending from these two.
> We need to set `Object.setPrototypeOf(...)` as TypeScript [introduced a breaking change](https://github.com/Microsoft/TypeScript-wiki/blob/main/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work) that may cause the inheritance to now work properly.
> We need to set `Object.setPrototypeOf(...)` as TypeScript [introduced a breaking change](https://github.com/Microsoft/TypeScript-wiki/blob/main/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work) that may cause the inheritance to now work properly.
## Handling errors
@ -59,7 +60,7 @@ I've come up with the following solution:
* Tuple of error and a result of an asynchronous task that might throw an error
*/
export type Either<ResultType, ErrorType extends Error> = Promise<
[error: ErrorType | null, result?: ResultType]
[error: null, result: ResultType] | [error: ErrorType, result: undefined]
>
/**
@ -102,7 +103,9 @@ As most of the errors will be possibly handled by the same `errorHandle` we can
* It will throw an error if any unexpected error is thrown
* When used with `wrapWithErrorHandler` it is able to catch most of the expected errors
*/
export function genericErrorHandler<ErrorType extends Error>(exception: ErrorType | unknown) {
export function genericErrorHandler<ErrorType extends Error>(
exception: ErrorType | unknown
) {
if (exception instanceof PublishableError) {
// These errors are usually handled no need to log them
return exception
@ -129,9 +132,10 @@ To spare some characters of redundant code I've applied a partial application pr
/**
* Helper function with applied `genericErrorHandler` to `wrapWithErrorHandler`
*/
export function wrapWithGenericErrorHandlerFunction<FunctionArgs extends Array<any>, ResultType>(
fn: (...args: FunctionArgs) => Promise<ResultType>
) {
export function wrapWithGenericErrorHandlerFunction<
FunctionArgs extends Array<any>,
ResultType
>(fn: (...args: FunctionArgs) => Promise<ResultType>) {
return wrapWithErrorHandler(genericErrorHandler, fn)
}
```
@ -141,11 +145,13 @@ export function wrapWithGenericErrorHandlerFunction<FunctionArgs extends Array<a
To sum it up, I'd like to present you the way how this API is used:
```typescript
const [error, memberId] = await wrapWithGenericErrorHandlerFunction(getUserId)(user.token)
if (error) {
await respond(error.message)
return
}
const [error, memberId] = await wrapWithGenericErrorHandlerFunction(getUserId)(
user.token
)
if (error) {
await respond(error.message)
return
}
```
This API allows me to keep the separation of concerns between the tasks and error handling into two separate functions. It also allows us to use the `wrapWithErrorHandler` with different `errorHandler` when we need to take special care. Also, I am still able to throw errors from the `errorHandler` that might be caught by a different `errorHandler` from an upper scope.