rudeshark.net/src/prelude/maybe.ts

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export interface Maybe<T> {
isJust(): this is Just<T>;
}
export type Just<T> = Maybe<T> & {
get(): T
};
export function just<T>(value: T): Just<T> {
return {
isJust: () => true,
get: () => value
};
}
export function nothing<T>(): Maybe<T> {
return {
isJust: () => false,
};
}