Fix AsyncDoEither errors

package.json

@@ -1,6 +1,6 @@
 {
   "name": "fp-lib",
-  "version": "1.0.1",
+  "version": "1.0.2",
   "description": "Zero-dependency functional programming library optimized for Bun",
   "main": "dist/index.js",
   "module": "dist/index.js",

src/either.ts

@@ -216,24 +216,28 @@ export class AsyncDoEither<T extends Record<string, unknown>, E> {
     f: (scope: T) => Either<A, E> | Promise<Either<A, E>>,
   ): AsyncDoEither<T & Record<K, A>, E> {
     const newPromise = this.promise.then(async (currentEither) => {
-      if (isLeft(currentEither)) {
+      // Propagate Left case as is
-        return currentEither as unknown as Either<T & Record<K, A>, E>
+      if (currentEither._tag === 'Left') {
+        return currentEither as Either<T & Record<K, A>, E>
       }
 
       const scope = currentEither.right
       try {
         const nextEither = await f(scope)
 
-        if (isLeft(nextEither)) {
+        // Propagate Left from bound computation
-          return nextEither as unknown as Either<T & Record<K, A>, E>
+        if (nextEither._tag === 'Left') {
+          return nextEither as Either<T & Record<K, A>, E>
         }
 
+        // Extend scope with new value
         return right({
           ...scope,
           [key]: nextEither.right,
         }) as Either<T & Record<K, A>, E>
       } catch (err) {
-        return left(err as E) as Either<T & Record<K, A>, E>
+        // Handle unexpected exceptions
+        return left(err as E)
       }
     })
 
@@ -248,8 +252,8 @@ export class AsyncDoEither<T extends Record<string, unknown>, E> {
    */
   return<B>(f: (scope: T) => B): Promise<Either<B, E>> {
     return this.promise.then(either => {
-      if (isLeft(either)) {
+      if (either._tag === 'Left') {
-        return either as unknown as Either<B, E>
+        return either as Either<B, E>
       }
       return right(f(either.right))
     })
@@ -283,15 +287,15 @@ export class AsyncDoEither<T extends Record<string, unknown>, E> {
       | Promise<Either<unknown, E>>): AsyncDoEither<T, E>
   {
     const newPromise = this.promise.then(async (currentEither) => {
-      if (isLeft(currentEither)) {
+      if (currentEither._tag === 'Left') {
         return currentEither
       }
 
       const scope = currentEither.right
       try {
         const effectEither = await f(scope)
-        if (isLeft(effectEither)) {
+        if (effectEither._tag === 'Left') {
-          return effectEither as unknown as Either<T, E>
+          return effectEither as Either<T, E>
         }
         return currentEither
       } catch (err) {