Data (Either)
The simplest example of @formulaic/fp will provide the functionality of an Either,
seen in Haskell,
and JavaScript libraries such as
fp-ts,
Folktale,
and more.
@formulaic/fp provides Data<T> and NoData<T>.
Data is intended to be used directly, but NoData is abstract
and usually implemented by more detailed classes, like NotFound.
Data<T> Interface
class Data<T> {
public readonly status: 200 | 201;
public readonly kind: "Data";
public readonly hasData: true;
public readonly noValue: false;
public readonly data: T;
public constructor(data: T) {}
}NoData<T> Interface
abstract class NoData<T> extends FP<T> {
public readonly hasData: false;
public readonly noValue: true;
}NotFound<T, EntityName> Interface
class NotFound<T, EntityName extends string> extends NoData<T> {
public readonly kind: "NotFound";
public readonly permissionError: boolean;
public readonly status: 403 | 404;
public readonly entityName?: EntityName;
public constructor(
entityName: EntityName,
permissionError: boolean,
) {}
}Usage
Use Data<T> when you may or may not have a return value:
function getLetterInString(haystack: string, index: number): Data<string> | EntityNotFound<string, "letter"> {
if(haystack.length < index) {
return new Data(haystack[index]);
}
return new EntityNotFound("letter", false);
}You can check hasData to determine the result:
const letter = getLetterInString("Hello World", 2);
if(letter.hasData) {
console.log(`Message contains '${letter.data}'`);
} else {
console.log("Could not extract the second letter from the message.");
}For most cases, there’s better ways to interact with FP data
than directly accessing fields, which will be covered next.