option-t/esm/undefinable/operators/map_async.js

A toolkit of Nullable/Option/Result type implementation in ECMAScript. Their APIs are inspired by Rust's `Option<T>` and `Result<T, E>`.

import { isUndefined, expectNotUndefined, } from '../core/undefinable.js';
import { ERR_MSG_TRANSFORMER_MUST_NOT_RETURN_NO_VAL_FOR_UNDEFINABLE } from '../internal/error_message.js';
/**
 *  Return the result of _transformer_ with using _input_ as an argument for it if _input_ is not `undefined`,
 *  Otherwise, return `undefined`.
 *
 *  * `U` must not be `Undefinable<*>`.
 *      * If you'd like return `Undefinable<*>` as `U`, use `andThen()`.
 *      * If the result of _transformer_ is `undefined`, this throw an `Error`.
 */
export async function mapAsyncForUndefinable(input, transformer) {
    if (isUndefined(input)) {
        return undefined;
    }
    const result = await transformer(input);
    // XXX:
    // If `U` is `Undefinable<SomeType>`, we think naturally the returned value of this function would be
    // the nested type `Undefinable<Undefinable<SomeType>>`. But this type means `(SomeType | undefined) | undefined`.
    // So a type checker would recognize this type as `SomeType | undefined`. So it's flattened.
    // Then the user should call `andThen` (_flatmap_) operation instead of this.
    const checked = expectNotUndefined(result, ERR_MSG_TRANSFORMER_MUST_NOT_RETURN_NO_VAL_FOR_UNDEFINABLE);
    return checked;
}