diff --git a/src/mapper/Mapper.ts b/src/mapper/Mapper.ts
index e87fd16..90ca4f2 100644
--- a/src/mapper/Mapper.ts
+++ b/src/mapper/Mapper.ts
@@ -1,9 +1,7 @@
-import { Either } from '@rimo/monad'
-
 /**
  * An interface for mappers functions that converts a source type `S` to target type `T`.
  *
  * @typeParam S the source type
  * @typeParam T the target type
  */
-export type Mapper<S, T> = <E extends Error>(source: S) => Either<E, T>
+export type Mapper<S, T> = (source: S) => T
diff --git a/src/repository/CrudRepository.ts b/src/repository/CrudRepository.ts
index 5e5c216..f780106 100644
--- a/src/repository/CrudRepository.ts
+++ b/src/repository/CrudRepository.ts
@@ -1,5 +1,4 @@
 import { Entity, EntityId } from '@rimo/core'
-import { Either } from '@rimo/monad'
 import { Repository } from './Repository'
 
 /**
@@ -20,7 +19,7 @@ export interface CrudRepository<T extends Entity<any>, ID extends EntityId>
    * @param entity to save.
    * @return the saved entity.
    */
-  save: <E extends Error, S extends T>(entity: S) => Promise<Either<E, S>>
+  save: <S extends T>(entity: S) => Promise<S>
 
   /**
    * Saves all given entities.
@@ -28,7 +27,7 @@ export interface CrudRepository<T extends Entity<any>, ID extends EntityId>
    * @param entities to save.
    * @return the saved entities.
    */
-  saveAll: <E extends Error, S extends T>(entities: S[]) => Promise<Either<E, S[]>>
+  saveAll: <S extends T>(entities: S[]) => Promise<S[]>
 
   /**
    * Retrieves an entity by its ID.
@@ -36,7 +35,7 @@ export interface CrudRepository<T extends Entity<any>, ID extends EntityId>
    * @param id of the entity.
    * @return the entity with the given ID or `undefined` if none found.
    */
-  findById: <E extends Error>(id: ID) => Promise<Either<E, T | undefined>>
+  findById: <E extends Error>(id: ID) => Promise<T | undefined>
 
   /**
    * Returns whether an entity with the given ID exists.
@@ -44,14 +43,14 @@ export interface CrudRepository<T extends Entity<any>, ID extends EntityId>
    * @param id of the entity.
    * @return `true` if an entity with the given id exists, `false` otherwise.
    */
-  existsById: <E extends Error>(id: ID) => Promise<Either<E, boolean>>
+  existsById: <E extends Error>(id: ID) => Promise<boolean>
 
   /**
    * Returns all instances of the type.
    *
    * @return all entities
    */
-  findAll: <E extends Error>() => Promise<Either<E, T[]>>
+  findAll: <E extends Error>() => Promise<T[]>
 
   /**
    * Returns all instances of the type `T` with the given IDs.
@@ -63,33 +62,33 @@ export interface CrudRepository<T extends Entity<any>, ID extends EntityId>
    * @param ids of entities to be retuned.
    * @return the entities.
    */
-  findAllById: <E extends Error>(ids: ID[]) => Promise<Either<E, T[]>>
+  findAllById: <E extends Error>(ids: ID[]) => Promise<T[]>
 
   /**
    * Returns the number of entities.
    *
    * @return the number of entities.
    */
-  count: <E extends Error>() => Promise<Either<E, number>>
+  count: <E extends Error>() => Promise<number>
 
   /**
    * Deletes the entity with the given id.
    *
    * @param id of entity to delete.
    */
-  deleteById: <E extends Error>(id: ID) => Promise<Either<E, void>>
+  deleteById: <E extends Error>(id: ID) => Promise<void>
 
   /**
    * Deletes a given entity.
    *
    * @param entity to delete.
    */
-  delete: <E extends Error>(entity: T) => Promise<Either<E, void>>
+  delete: <E extends Error>(entity: T) => Promise<void>
 
   /**
    * Deletes the given entities, or all when no arguments are provided.
    *
    * @param entities to delete or `undefined` to delete all.
    */
-  deleteAll: <E extends Error>(entities?: T[]) => Promise<Either<E, void>>
+  deleteAll: <E extends Error>(entities?: T[]) => Promise<void>
 }