Effect CommunityEC
Effect Community•2y ago
bigpopakap

Modeling a Value with Possible Errors in TypeScript

I'm curious how best to model this, i.e. if there are any built-in types to model "a value, with maybe some errors"

I have a cache of all the data my frontend has loaded:
type MyData = // some type of data
type MyError = // network error, or parsing error, or whatever. some discriminated union type

// I want to model something like "the last value fetched was A and maybe it's stale that's ok. The last fetch optionally had some errors
// So any use case can always rely on showing the user *some value*, and can optionally choose to care about the errors (including to figure out whether the value is stale)
type ValueAndMaybeError<A, E> = // ??

cache: Record<string, ValueAndMaybeError<MyData, MyError>>


In particular, I want to model this so it's easy to "merge in" the results of the next fetch:
// functions to fetch one, or many, or whatever
declare const fetchOne: (...) => Effect<MyData, MyError, never>
declare const fetchMany: (...) => Effect<Record<string, MyData>, MyError, never>;

// run any fetch call to get the Either result of the call
const result = yield* Effect.either(fetchOneOrMany());

// Now I want this part to be easy
const updateValue = (
  existing: ValueAndMaybeError<MyData, MyError>,
  result: Either<MyData, MyError>
) : ValueAndMaybeError<MyData, MyError> => {
   // if result is success, return ValueAndMaybeError with MyData and clear the error

   // if result is failure, return ValueAndMaybeError with the existing value, and replace any error from before with the one from result
}


I could probably force it with a Tuple or discriminated union or just simply { value: A, error: Option<E> }, and just implement all this pseudocode... but I suspect that the correct choice of types will simplify this a lot
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