Add Tag type and hasTag function

src/lib/util/basic/either.ts

@@ -22,7 +22,7 @@ export const eitherChain =
     fa._tag === 'Right' ? f(fa.right) : fa
 
 export const fold =
-  <A, B, E>(onLeft: (e: E) => B, onRight: (a: A) => B) =>
+  <A, E, B>(onLeft: (e: E) => B, onRight: (a: A) => B) =>
   (fa: Either<A, E>): B =>
     fa._tag === 'Left' ? onLeft(fa.left) : onRight(fa.right)
 

src/lib/util/basic/utility.ts

@@ -174,5 +174,11 @@ export const tryCatch = <A>(f: () => A): Either<A, Error> => {
   }
 }
 
+export interface Tag<T> {
+  readonly _tag: T
+}
+
+export const hasTag = (x: string) => (y: Tag<string>) => x === y._tag
+
 export const fromNullable = <A>(a: A | null | undefined): Option<A> =>
   a == null ? none : some(a)