diff --git a/README.md b/README.md
index be6005d..a1dbb63 100644
--- a/README.md
+++ b/README.md
@@ -217,19 +217,18 @@ user-facing errors.
 #### Fetch customer information
 
 ```
-$ curl -i -X GET  'http://localhost:9000/customers/5'
+$ curl -i -X GET  'http://localhost:9000/customers/1'
 ```
 
 ```
 HTTP/1.1 200 OK
 Content-Type: application/json; charset=UTF-8
-Content-Length: 93
+Content-Length: 84
 
 {
-  "id": 5,
   "firstName": "Michael",
   "lastName": "Bull",
-  "email": "example@email.com"
+  "email": "michael@example.com"
 }
 ```
 
@@ -242,7 +241,7 @@ $ curl -i -X POST \
 '{
   "firstName": "Your",
   "lastName": "Name",
-  "email": "your@email.com"
+  "email": "email@example.com"
 }' \
  'http://localhost:9000/customers/200'
 ```
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/Application.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/Application.kt
index fd70dea..18bcb42 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/Application.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/Application.kt
@@ -1,11 +1,8 @@
 package com.github.michaelbull.result.example
 
-import com.github.michaelbull.result.Ok
 import com.github.michaelbull.result.Result
 import com.github.michaelbull.result.andThen
-import com.github.michaelbull.result.example.model.domain.Customer
-import com.github.michaelbull.result.example.model.domain.CustomerCreated
-import com.github.michaelbull.result.example.model.domain.CustomerId
+import com.github.michaelbull.result.example.model.domain.Created
 import com.github.michaelbull.result.example.model.domain.CustomerIdMustBePositive
 import com.github.michaelbull.result.example.model.domain.CustomerNotFound
 import com.github.michaelbull.result.example.model.domain.CustomerRequired
@@ -16,15 +13,20 @@ import com.github.michaelbull.result.example.model.domain.EmailAddressChanged
 import com.github.michaelbull.result.example.model.domain.EmailInvalid
 import com.github.michaelbull.result.example.model.domain.EmailRequired
 import com.github.michaelbull.result.example.model.domain.EmailTooLong
+import com.github.michaelbull.result.example.model.domain.Event
+import com.github.michaelbull.result.example.model.domain.FirstNameChanged
 import com.github.michaelbull.result.example.model.domain.FirstNameRequired
 import com.github.michaelbull.result.example.model.domain.FirstNameTooLong
+import com.github.michaelbull.result.example.model.domain.LastNameChanged
 import com.github.michaelbull.result.example.model.domain.LastNameRequired
 import com.github.michaelbull.result.example.model.domain.LastNameTooLong
 import com.github.michaelbull.result.example.model.domain.SqlCustomerInvalid
 import com.github.michaelbull.result.example.model.dto.CustomerDto
+import com.github.michaelbull.result.example.model.entity.CustomerEntity
+import com.github.michaelbull.result.example.model.entity.CustomerId
+import com.github.michaelbull.result.example.repository.InMemoryCustomerRepository
 import com.github.michaelbull.result.example.service.CustomerService
 import com.github.michaelbull.result.mapBoth
-import com.github.michaelbull.result.mapError
 import com.github.michaelbull.result.toResultOr
 import io.ktor.application.Application
 import io.ktor.application.call
@@ -52,56 +54,49 @@ fun Application.main() {
         }
     }
 
+    val customers = setOf(
+        CustomerEntity(CustomerId(1L), "Michael", "Bull", "michael@example.com"),
+        CustomerEntity(CustomerId(2L), "Kevin", "Herron", "kevin@example.com"),
+        CustomerEntity(CustomerId(3L), "Markus", "Padourek", "markus@example.com"),
+        CustomerEntity(CustomerId(4L), "Tristan", "Hamilton", "tristan@example.com"),
+    )
+
+    val customersById = customers.associateBy(CustomerEntity::id).toMutableMap()
+    val customerRepository = InMemoryCustomerRepository(customersById)
+    val customerService = CustomerService(customerRepository)
+
     routing {
         get("/customers/{id}") {
-            call.parameters.readId()
-                .andThen(CustomerId.Companion::create)
-                .andThen(CustomerService::getById)
-                .mapError(::messageToResponse)
-                .mapBoth(
-                    success = { customer ->
-                        call.respond(HttpStatusCode.OK, CustomerDto.from(customer))
-                    },
-                    failure = { (status, message) ->
-                        call.respond(status, message)
-                    }
-                )
+            val (status, message) = call.parameters.readId()
+                .andThen(customerService::getById)
+                .mapBoth(::customerToResponse, ::messageToResponse)
+
+            call.respond(status, message)
         }
 
         post("/customers/{id}") {
-            call.parameters.readId()
-                .andThen { id ->
-                    val dto = call.receive<CustomerDto>()
-                    dto.id = id
-                    Ok(dto)
-                }
-                .andThen(Customer.Companion::from)
-                .andThen(CustomerService::upsert)
-                .mapError(::messageToResponse)
-                .mapBoth(
-                    success = { event ->
-                        if (event == null) {
-                            call.respond(HttpStatusCode.NotModified)
-                        } else {
-                            val (status, message) = messageToResponse(event)
-                            call.respond(status, message)
-                        }
-                    },
-                    failure = { (status, message) ->
-                        call.respond(status, message)
-                    }
-                )
+            val (status, message) = call.parameters.readId()
+                .andThen { customerService.save(it, call.receive()) }
+                .mapBoth(::eventToResponse, ::messageToResponse)
+
+            if (message != null) {
+                call.respond(status, message)
+            } else {
+                call.respond(status)
+            }
         }
     }
 
 }
 
 private fun Parameters.readId(): Result<Long, DomainMessage> {
-    return this["id"]
+    return get("id")
         ?.toLongOrNull()
         .toResultOr { CustomerRequired }
 }
 
+private fun customerToResponse(customer: CustomerDto) = HttpStatusCode.OK to customer
+
 private fun messageToResponse(message: DomainMessage) = when (message) {
     CustomerRequired,
     CustomerIdMustBePositive,
@@ -112,23 +107,32 @@ private fun messageToResponse(message: DomainMessage) = when (message) {
     EmailRequired,
     EmailTooLong,
     EmailInvalid ->
-        Pair(HttpStatusCode.BadRequest, "There is an error in your request")
-
-// events
-    CustomerCreated ->
-        Pair(HttpStatusCode.Created, "Customer created")
-
-    is EmailAddressChanged ->
-        Pair(HttpStatusCode.OK, "Email address changed from ${message.old} to ${message.new}")
+        HttpStatusCode.BadRequest to "There is an error in your request"
 
 // exposed errors
     CustomerNotFound ->
-        Pair(HttpStatusCode.NotFound, "Unknown customer")
+        HttpStatusCode.NotFound to "Unknown customer"
 
 // internal errors
     SqlCustomerInvalid,
     DatabaseTimeout,
     is DatabaseError ->
-        Pair(HttpStatusCode.InternalServerError, "Internal server error occurred")
-
+        HttpStatusCode.InternalServerError to "Internal server error occurred"
+}
+
+private fun eventToResponse(event: Event?) = when (event) {
+    null ->
+        HttpStatusCode.NotModified to null
+
+    Created ->
+        HttpStatusCode.Created to "Customer created"
+
+    is FirstNameChanged ->
+        HttpStatusCode.OK to "First name changed from ${event.old} to ${event.new}"
+
+    is LastNameChanged ->
+        HttpStatusCode.OK to "First name changed from ${event.old} to ${event.new}"
+
+    is EmailAddressChanged ->
+        HttpStatusCode.OK to "Email address changed from ${event.old} to ${event.new}"
 }
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Customer.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Customer.kt
index 4e4aeba..32ec51b 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Customer.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Customer.kt
@@ -1,28 +1,6 @@
 package com.github.michaelbull.result.example.model.domain
 
-import com.github.michaelbull.result.Result
-import com.github.michaelbull.result.example.model.dto.CustomerDto
-import com.github.michaelbull.result.example.model.entity.CustomerEntity
-import com.github.michaelbull.result.zip
-
 data class Customer(
-    val id: CustomerId,
     val name: PersonalName,
     val email: EmailAddress
-) {
-    companion object {
-        fun from(entity: CustomerEntity): Result<Customer, DomainMessage> {
-            val createId = { CustomerId.create(entity.id) }
-            val createName = { PersonalName.create(entity.firstName, entity.lastName) }
-            val createEmail = { EmailAddress.create(entity.email) }
-            return zip(createId, createName, createEmail, ::Customer)
-        }
-
-        fun from(dto: CustomerDto): Result<Customer, DomainMessage> {
-            val createId = { CustomerId.create(dto.id) }
-            val createName = { PersonalName.create(dto.firstName, dto.lastName) }
-            val createEmail = { EmailAddress.create(dto.email) }
-            return zip(createId, createName, createEmail, ::Customer)
-        }
-    }
-}
+)
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/CustomerId.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/CustomerId.kt
deleted file mode 100644
index fcfb5f5..0000000
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/CustomerId.kt
+++ /dev/null
@@ -1,14 +0,0 @@
-package com.github.michaelbull.result.example.model.domain
-
-import com.github.michaelbull.result.Err
-import com.github.michaelbull.result.Ok
-
-data class CustomerId(val id: Long) {
-    companion object {
-        fun create(id: Long?) = when {
-            id == null -> Err(CustomerRequired)
-            id < 1 -> Err(CustomerIdMustBePositive)
-            else -> Ok(CustomerId(id))
-        }
-    }
-}
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/DomainMessage.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/DomainMessage.kt
index e867dec..832b48c 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/DomainMessage.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/DomainMessage.kt
@@ -20,11 +20,6 @@ object EmailRequired : DomainMessage()
 object EmailTooLong : DomainMessage()
 object EmailInvalid : DomainMessage()
 
-/* events */
-
-object CustomerCreated : DomainMessage()
-class EmailAddressChanged(val old: String, val new: String) : DomainMessage()
-
 /* exposed errors */
 
 object CustomerNotFound : DomainMessage()
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/EmailAddress.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/EmailAddress.kt
index 0ec6944..407ba20 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/EmailAddress.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/EmailAddress.kt
@@ -1,24 +1,5 @@
 package com.github.michaelbull.result.example.model.domain
 
-import com.github.michaelbull.result.Err
-import com.github.michaelbull.result.Ok
-import com.github.michaelbull.result.Result
-
 data class EmailAddress(
     val address: String
-) {
-    companion object {
-        private const val MAX_LENGTH = 20
-        private val PATTERN = ".+@.+\\..+".toRegex() // crude validation
-
-        fun create(address: String?): Result<EmailAddress, DomainMessage> {
-            return when {
-                address.isNullOrBlank() -> Err(EmailRequired)
-                address.length > MAX_LENGTH -> Err(EmailTooLong)
-                !address.matches(PATTERN) -> Err(EmailInvalid)
-                else -> Ok(EmailAddress(address))
-            }
-        }
-    }
-}
-
+)
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Event.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Event.kt
new file mode 100644
index 0000000..56527fd
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/Event.kt
@@ -0,0 +1,8 @@
+package com.github.michaelbull.result.example.model.domain
+
+sealed class Event
+
+object Created : Event()
+class FirstNameChanged(val old: String, val new: String) : Event()
+class LastNameChanged(val old: String, val new: String) : Event()
+class EmailAddressChanged(val old: String, val new: String) : Event()
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/PersonalName.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/PersonalName.kt
index ae6564e..710ee0b 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/PersonalName.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/PersonalName.kt
@@ -7,18 +7,18 @@ import com.github.michaelbull.result.Result
 data class PersonalName(
     val first: String,
     val last: String
-) {
-    companion object {
-        private const val MAX_LENGTH = 10
+)
 
-        fun create(first: String?, last: String?): Result<PersonalName, DomainMessage> {
-            return when {
-                first.isNullOrBlank() -> Err(FirstNameRequired)
-                last.isNullOrBlank() -> Err(LastNameRequired)
-                first.length > MAX_LENGTH -> Err(FirstNameTooLong)
-                last.length > MAX_LENGTH -> Err(LastNameTooLong)
-                else -> Ok(PersonalName(first, last))
-            }
-        }
+private const val MAX_LENGTH = 10
+
+fun Pair<String?, String?>.toPersonalName(): Result<PersonalName, DomainMessage> {
+    val (first, last) = this
+
+    return when {
+        first.isNullOrBlank() -> Err(FirstNameRequired)
+        last.isNullOrBlank() -> Err(LastNameRequired)
+        first.length > MAX_LENGTH -> Err(FirstNameTooLong)
+        last.length > MAX_LENGTH -> Err(LastNameTooLong)
+        else -> Ok(PersonalName(first, last))
     }
 }
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/CustomerRepository.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/CustomerRepository.kt
deleted file mode 100644
index 0fb7656..0000000
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/CustomerRepository.kt
+++ /dev/null
@@ -1,9 +0,0 @@
-package com.github.michaelbull.result.example.model.domain.repository
-
-import com.github.michaelbull.result.example.model.domain.Customer
-import com.github.michaelbull.result.example.model.entity.CustomerEntity
-
-/**
- * A repository that stores [Customers][Customer] with a [Long] ID.
- */
-interface CustomerRepository : Repository<CustomerEntity, Long>
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/dto/CustomerDto.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/dto/CustomerDto.kt
index 70c5a88..70b4ecf 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/dto/CustomerDto.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/dto/CustomerDto.kt
@@ -1,23 +1,7 @@
 package com.github.michaelbull.result.example.model.dto
 
-import com.github.michaelbull.result.example.model.domain.Customer
-
-/**
- * A [DTO](https://en.wikipedia.org/wiki/Data_transfer_object) sent over the network
- * that represents a [Customer].
- */
 data class CustomerDto(
-    var id: Long = 0L,
-    var firstName: String? = null,
-    var lastName: String? = null,
-    var email: String? = null
-) {
-    companion object {
-        fun from(customer: Customer) = CustomerDto(
-            id = customer.id.id,
-            firstName = customer.name.first,
-            lastName = customer.name.last,
-            email = customer.email.address
-        )
-    }
-}
+    val firstName: String,
+    val lastName: String,
+    val email: String
+)
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerEntity.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerEntity.kt
index 1ea6b2d..6b72424 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerEntity.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerEntity.kt
@@ -1,23 +1,12 @@
 package com.github.michaelbull.result.example.model.entity
 
-import com.github.michaelbull.result.example.model.domain.Customer
-
 /**
  * Represents an [Entity](https://docs.oracle.com/cd/E17277_02/html/collections/tutorial/Entity.html)
  * mapped to a table in a database.
  */
 data class CustomerEntity(
-    var id: Long = 0L,
-    var firstName: String? = null,
-    var lastName: String? = null,
-    var email: String? = null
-) {
-    companion object {
-        fun from(customer: Customer) = CustomerEntity(
-            id = customer.id.id,
-            firstName = customer.name.first,
-            lastName = customer.name.last,
-            email = customer.email.address
-        )
-    }
-}
+    val id: CustomerId,
+    val firstName: String,
+    val lastName: String,
+    val email: String
+)
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerId.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerId.kt
new file mode 100644
index 0000000..587f3aa
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/model/entity/CustomerId.kt
@@ -0,0 +1,3 @@
+package com.github.michaelbull.result.example.model.entity
+
+data class CustomerId(val id: Long)
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/repository/CustomerRepository.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/CustomerRepository.kt
new file mode 100644
index 0000000..3883b33
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/CustomerRepository.kt
@@ -0,0 +1,9 @@
+package com.github.michaelbull.result.example.repository
+
+import com.github.michaelbull.result.example.model.entity.CustomerEntity
+import com.github.michaelbull.result.example.model.entity.CustomerId
+
+/**
+ * A repository that stores a [CustomerEntity] identified by a [CustomerId].
+ */
+interface CustomerRepository : Repository<CustomerEntity, CustomerId>
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/repository/InMemoryCustomerRepository.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/InMemoryCustomerRepository.kt
new file mode 100644
index 0000000..1fdeb18
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/InMemoryCustomerRepository.kt
@@ -0,0 +1,28 @@
+package com.github.michaelbull.result.example.repository
+
+import com.github.michaelbull.result.example.model.entity.CustomerEntity
+import com.github.michaelbull.result.example.model.entity.CustomerId
+import java.sql.SQLTimeoutException
+
+class InMemoryCustomerRepository(
+    private val customers: MutableMap<CustomerId, CustomerEntity>
+) : CustomerRepository {
+
+    override fun findById(id: CustomerId): CustomerEntity? {
+        return customers.entries.find { (key) -> key == id }?.value
+    }
+
+    override fun save(entity: CustomerEntity) {
+        val id = entity.id
+
+        if (id == TIMEOUT_CUSTOMER_ID) {
+            throw SQLTimeoutException()
+        } else {
+            customers[id] = entity
+        }
+    }
+
+    private companion object {
+        private val TIMEOUT_CUSTOMER_ID = CustomerId(42L)
+    }
+}
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/Repository.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/Repository.kt
similarity index 50%
rename from example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/Repository.kt
rename to example/src/main/kotlin/com/github/michaelbull/result/example/repository/Repository.kt
index e22844e..3e9b96a 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/model/domain/repository/Repository.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/repository/Repository.kt
@@ -1,10 +1,9 @@
-package com.github.michaelbull.result.example.model.domain.repository
+package com.github.michaelbull.result.example.repository
 
 /**
  * A class that encapsulates storage and retrieval of domain objects of type [T], identified by a key of type [ID].
  */
 interface Repository<T, ID> {
-    fun findAll(): Collection<T>
-    fun update(entity: T)
-    fun insert(entity: T)
+    fun findById(id: ID): T?
+    fun save(entity: T)
 }
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt
index 34b4d46..7e1a9df 100644
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt
@@ -1,69 +1,126 @@
 package com.github.michaelbull.result.example.service
 
+import com.github.michaelbull.result.Err
+import com.github.michaelbull.result.Ok
 import com.github.michaelbull.result.Result
 import com.github.michaelbull.result.andThen
+import com.github.michaelbull.result.example.model.domain.Created
 import com.github.michaelbull.result.example.model.domain.Customer
-import com.github.michaelbull.result.example.model.domain.CustomerCreated
-import com.github.michaelbull.result.example.model.domain.CustomerId
+import com.github.michaelbull.result.example.model.domain.CustomerIdMustBePositive
 import com.github.michaelbull.result.example.model.domain.CustomerNotFound
+import com.github.michaelbull.result.example.model.domain.CustomerRequired
 import com.github.michaelbull.result.example.model.domain.DatabaseError
 import com.github.michaelbull.result.example.model.domain.DatabaseTimeout
 import com.github.michaelbull.result.example.model.domain.DomainMessage
 import com.github.michaelbull.result.example.model.domain.EmailAddressChanged
+import com.github.michaelbull.result.example.model.domain.Event
+import com.github.michaelbull.result.example.model.domain.FirstNameChanged
+import com.github.michaelbull.result.example.model.domain.LastNameChanged
+import com.github.michaelbull.result.example.model.dto.CustomerDto
 import com.github.michaelbull.result.example.model.entity.CustomerEntity
+import com.github.michaelbull.result.example.model.entity.CustomerId
+import com.github.michaelbull.result.example.repository.CustomerRepository
+import com.github.michaelbull.result.getOrElse
 import com.github.michaelbull.result.map
-import com.github.michaelbull.result.mapAll
-import com.github.michaelbull.result.mapBoth
 import com.github.michaelbull.result.mapError
 import com.github.michaelbull.result.runCatching
 import com.github.michaelbull.result.toResultOr
+import com.github.michaelbull.result.zip
 import java.sql.SQLTimeoutException
 
-object CustomerService {
-    private val repository = InMemoryCustomerRepository()
+class CustomerService(
+    private val repository: CustomerRepository
+) {
 
-    fun getAll(): Result<Collection<Customer>, DomainMessage> {
-        return runCatching(repository::findAll)
-            .mapError(::exceptionToDomainMessage)
-            .mapAll(Customer.Companion::from)
+    fun getById(id: Long): Result<CustomerDto, DomainMessage> {
+        return parseCustomerId(id)
+            .andThen(::findById)
+            .map(::entityToDto)
     }
 
-    fun getById(id: CustomerId): Result<Customer, DomainMessage> {
-        return getAll().andThen { customers -> customers.findCustomer(id) }
+    fun save(id: Long, dto: CustomerDto): Result<Event?, DomainMessage> {
+        return parseCustomerId(id)
+            .andThen { upsert(it, dto) }
     }
 
-    fun upsert(customer: Customer): Result<DomainMessage?, DomainMessage> {
-        val entity = CustomerEntity.from(customer)
-        return getById(customer.id).mapBoth(
-            success = { existing -> updateCustomer(entity, existing, customer) },
-            failure = { createCustomer(entity) }
+    private fun parseCustomerId(id: Long?) = when {
+        id == null -> Err(CustomerRequired)
+        id < 1 -> Err(CustomerIdMustBePositive)
+        else -> Ok(CustomerId(id))
+    }
+
+    private fun entityToDto(entity: CustomerEntity): CustomerDto {
+        return CustomerDto(
+            firstName = entity.firstName,
+            lastName = entity.lastName,
+            email = entity.email
         )
     }
 
-    private fun updateCustomer(entity: CustomerEntity, old: Customer, new: Customer) =
-        runCatching { repository.update(entity) }
-            .map { differenceBetween(old, new) }
-            .mapError(::exceptionToDomainMessage)
-
-    private fun createCustomer(entity: CustomerEntity) =
-        runCatching { repository.insert(entity) }
-            .map { CustomerCreated }
-            .mapError(::exceptionToDomainMessage)
-
-    private fun Collection<Customer>.findCustomer(id: CustomerId): Result<Customer, CustomerNotFound> {
-        return find { it.id == id }.toResultOr { CustomerNotFound }
+    private fun findById(id: CustomerId): Result<CustomerEntity, CustomerNotFound> {
+        return repository.findById(id)
+            .toResultOr { CustomerNotFound }
     }
 
-    private fun differenceBetween(old: Customer, new: Customer): EmailAddressChanged? {
-        return if (new.email != old.email) {
-            EmailAddressChanged(old.email.address, new.email.address)
+    private fun upsert(id: CustomerId, dto: CustomerDto): Result<Event?, DomainMessage> {
+        val existingCustomer = repository.findById(id)
+
+        return if (existingCustomer != null) {
+            update(existingCustomer, dto)
         } else {
-            null
+            insert(id, dto)
         }
     }
 
+    private fun update(entity: CustomerEntity, dto: CustomerDto): Result<Event?, DomainMessage> {
+        val validated = validate(dto).getOrElse { return Err(it) }
+
+        val updated = entity.copy(
+            firstName = validated.name.first,
+            lastName = validated.name.last,
+            email = validated.email.address
+        )
+
+        return runCatching { repository.save(updated) }
+            .map { compare(entity, updated) }
+            .mapError(::exceptionToDomainMessage)
+    }
+
+    private fun insert(id: CustomerId, dto: CustomerDto): Result<Created, DomainMessage> {
+        val entity = createEntity(id, dto).getOrElse { return Err(it) }
+
+        return runCatching { repository.save(entity) }
+            .map { Created }
+            .mapError(::exceptionToDomainMessage)
+    }
+
+    private fun validate(dto: CustomerDto): Result<Customer, DomainMessage> {
+        return zip(
+            { PersonalNameParser.parse(dto.firstName, dto.lastName) },
+            { EmailAddressParser.parse(dto.email) },
+            ::Customer
+        )
+    }
+
+    private fun createEntity(id: CustomerId, dto: CustomerDto): Result<CustomerEntity, DomainMessage> {
+        return zip(
+            { PersonalNameParser.parse(dto.firstName, dto.lastName) },
+            { EmailAddressParser.parse(dto.email) },
+            { (first, last), (address) -> CustomerEntity(id, first, last, address) }
+        )
+    }
+
     private fun exceptionToDomainMessage(t: Throwable) = when (t) {
         is SQLTimeoutException -> DatabaseTimeout
         else -> DatabaseError(t.message)
     }
+
+    private fun compare(old: CustomerEntity, new: CustomerEntity): Event? {
+        return when {
+            new.firstName != old.firstName -> FirstNameChanged(old.firstName, new.firstName)
+            new.lastName != old.lastName -> LastNameChanged(old.lastName, new.lastName)
+            new.email != old.email -> EmailAddressChanged(old.email, new.email)
+            else -> null
+        }
+    }
 }
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/service/EmailAddressParser.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/service/EmailAddressParser.kt
new file mode 100644
index 0000000..f929825
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/service/EmailAddressParser.kt
@@ -0,0 +1,25 @@
+package com.github.michaelbull.result.example.service
+
+import com.github.michaelbull.result.Err
+import com.github.michaelbull.result.Ok
+import com.github.michaelbull.result.Result
+import com.github.michaelbull.result.example.model.domain.DomainMessage
+import com.github.michaelbull.result.example.model.domain.EmailAddress
+import com.github.michaelbull.result.example.model.domain.EmailInvalid
+import com.github.michaelbull.result.example.model.domain.EmailRequired
+import com.github.michaelbull.result.example.model.domain.EmailTooLong
+
+object EmailAddressParser {
+
+    private const val MAX_LENGTH = 20
+    private val PATTERN = ".+@.+\\..+".toRegex() // crude validation
+
+    fun parse(address: String?): Result<EmailAddress, DomainMessage> {
+        return when {
+            address.isNullOrBlank() -> Err(EmailRequired)
+            address.length > MAX_LENGTH -> Err(EmailTooLong)
+            !address.matches(this.PATTERN) -> Err(EmailInvalid)
+            else -> Ok(EmailAddress(address))
+        }
+    }
+}
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/service/InMemoryCustomerRepository.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/service/InMemoryCustomerRepository.kt
deleted file mode 100644
index 3f828d0..0000000
--- a/example/src/main/kotlin/com/github/michaelbull/result/example/service/InMemoryCustomerRepository.kt
+++ /dev/null
@@ -1,44 +0,0 @@
-package com.github.michaelbull.result.example.service
-
-import com.github.michaelbull.result.example.model.domain.repository.CustomerRepository
-import com.github.michaelbull.result.example.model.entity.CustomerEntity
-import java.sql.SQLException
-import java.sql.SQLTimeoutException
-
-class InMemoryCustomerRepository : CustomerRepository {
-    private val table = mutableMapOf(
-        5L to CustomerEntity(5L, "Michael", "Bull", "example@email.com")
-    )
-
-    override fun findAll(): Collection<CustomerEntity> {
-        return table.values
-    }
-
-    override fun update(entity: CustomerEntity) {
-        val id = entity.id
-
-        if (id !in table) {
-            throw SQLException("No customer found for id $id")
-        } else {
-            setOrTimeout(id, entity)
-        }
-    }
-
-    override fun insert(entity: CustomerEntity) {
-        val id = entity.id
-
-        if (id in table) {
-            throw SQLException("Customer already exists with id $id")
-        } else {
-            setOrTimeout(id, entity)
-        }
-    }
-
-    private fun setOrTimeout(id: Long, entity: CustomerEntity) {
-        if (id == 42L) {
-            throw SQLTimeoutException()
-        } else {
-            table[id] = entity
-        }
-    }
-}
diff --git a/example/src/main/kotlin/com/github/michaelbull/result/example/service/PersonalNameParser.kt b/example/src/main/kotlin/com/github/michaelbull/result/example/service/PersonalNameParser.kt
new file mode 100644
index 0000000..1759739
--- /dev/null
+++ b/example/src/main/kotlin/com/github/michaelbull/result/example/service/PersonalNameParser.kt
@@ -0,0 +1,26 @@
+package com.github.michaelbull.result.example.service
+
+import com.github.michaelbull.result.Err
+import com.github.michaelbull.result.Ok
+import com.github.michaelbull.result.Result
+import com.github.michaelbull.result.example.model.domain.DomainMessage
+import com.github.michaelbull.result.example.model.domain.FirstNameRequired
+import com.github.michaelbull.result.example.model.domain.FirstNameTooLong
+import com.github.michaelbull.result.example.model.domain.LastNameRequired
+import com.github.michaelbull.result.example.model.domain.LastNameTooLong
+import com.github.michaelbull.result.example.model.domain.PersonalName
+
+object PersonalNameParser {
+
+    private const val MAX_LENGTH = 10
+
+    fun parse(first: String?, last: String?): Result<PersonalName, DomainMessage> {
+        return when {
+            first.isNullOrBlank() -> Err(FirstNameRequired)
+            last.isNullOrBlank() -> Err(LastNameRequired)
+            first.length > MAX_LENGTH -> Err(FirstNameTooLong)
+            last.length > MAX_LENGTH -> Err(LastNameTooLong)
+            else -> Ok(PersonalName(first, last))
+        }
+    }
+}