Add toResultOr

Acts as a factory function to convert nullable types to Result types

README.md

@@ -52,12 +52,24 @@ Oriented Programming is to avoid throwing an exception and instead make the
 return type of your function a `Result`.
 
 ```kotlin
-fun findById(id: Int): Result<Customer, DatabaseError> {
-    val customer = getAllCustomers().find { it.id == id }
-    return if (customer != null) Ok(customer) else Err(DatabaseError.CustomerNotFound)
+fun checkPrivileges(user: User, command: Command): Result<Command, CommandError> {
+    return if (user.rank >= command.mininimumRank) {
+        Ok(command)
+    } else {
+        Err(CommandError.InsufficientRank(command.name))
+    }
 }
 ```
 
+Nullable types, such as the `find` method in the example below, can be 
+converted to a `Result` using the `toResultOr` extension function.
+
+```kotlin
+val result: Result<Customer, String> = customers
+    .find { it.id == id } // returns Customer?
+    .toResultOr { "No customer found" }
+```
+
 ### Transforming Results
 
 Both success and failure results can be transformed within a stage of the
@@ -68,7 +80,7 @@ program error (`UnlockError`) into an exposed client error
 ```kotlin
 val result: Result<Treasure, UnlockResponse> = 
     unlockVault("my-password") // returns Result<Treasure, UnlockError>
-    .mapError { UnlockResponse.IncorrectPassword } // transform UnlockError into UnlockResponse.IncorrectPassword
+    .mapError { IncorrectPassword } // transform UnlockError into IncorrectPassword
 ```
 
 ### Chaining
@@ -77,6 +89,7 @@ Results can be chained to produce a "happy path" of execution. For example, the
 happy path for a user entering commands into an administrative console would 
 consist of: the command being tokenized, the command being registered, the user
 having sufficient privileges, and the command executing the associated action.
+The example below uses the `checkPrivileges` function we defined earlier.
 
 ```kotlin
 tokenize(command.toLowerCase())
@@ -89,17 +102,6 @@ tokenize(command.toLowerCase())
     )
 ```
 
-Each of the `andThen` steps produces its own result, for example:
-
-```kotlin
-fun checkPrivileges(user: User, command: TokenizedCommand): Result<Command, CommandError> {
-    return when {
-        user.rank >= command.minRank -> Ok(command)
-        else -> Err(CommandError.InsufficientRank(command.tokens.name))
-    }
-}
-``` 
-
 ## Inspiration
 
 Inspiration for this library has been drawn from other languages in which the

example/src/main/kotlin/com/github/michaelbull/result/example/Application.kt

@@ -1,6 +1,5 @@
 package com.github.michaelbull.result.example
 
-import com.github.michaelbull.result.Err
 import com.github.michaelbull.result.Ok
 import com.github.michaelbull.result.Result
 import com.github.michaelbull.result.andThen
@@ -26,6 +25,7 @@ import com.github.michaelbull.result.example.model.dto.CustomerDto
 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
 import io.ktor.application.install
@@ -91,8 +91,9 @@ fun Application.main() {
 }
 
 private fun readId(values: ValuesMap): Result<Long, DomainMessage> {
-    val id = values["id"]?.toLongOrNull()
-    return if (id != null) Ok(id) else Err(CustomerRequired)
+    return values["id"]
+        ?.toLongOrNull()
+        .toResultOr { CustomerRequired }
 }
 
 private fun messageToResponse(message: DomainMessage) = when (message) {

example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt

@@ -16,6 +16,7 @@ import com.github.michaelbull.result.example.model.entity.CustomerEntity
 import com.github.michaelbull.result.map
 import com.github.michaelbull.result.mapBoth
 import com.github.michaelbull.result.mapError
+import com.github.michaelbull.result.toResultOr
 import java.sql.SQLTimeoutException
 
 object CustomerService {
@@ -36,7 +37,7 @@ object CustomerService {
     }
 
     fun getById(id: CustomerId): Result<Customer, DomainMessage> {
-        return getAll().andThen { findCustomer(it, id) }
+        return getAll().andThen { it.findCustomer(id) }
     }
 
     fun upsert(customer: Customer): Result<DomainMessage?, DomainMessage> {
@@ -57,9 +58,8 @@ object CustomerService {
             .map { CustomerCreated }
             .mapError(::exceptionToDomainMessage)
 
-    private fun findCustomer(customers: Collection<Customer>, id: CustomerId): Result<Customer, CustomerNotFound> {
-        val customer = customers.find { it.id == id }
-        return if (customer != null) Ok(customer) else Err(CustomerNotFound)
+    private fun Collection<Customer>.findCustomer(id: CustomerId): Result<Customer, CustomerNotFound> {
+        return find { it.id == id }.toResultOr { CustomerNotFound }
     }
 
     private fun differenceBetween(old: Customer, new: Customer): EmailAddressChanged? {

src/main/kotlin/com/github/michaelbull/result/Result.kt

@@ -11,10 +11,10 @@ sealed class Result<out V, out E> {
     companion object {
 
         /**
-         * Invokes a [function] and wraps it in a [Result], returning an [Err] if an [Exception]
-         * was thrown, otherwise [Ok].
+         * Invokes a [function] and wraps it in a [Result], returning an [Err]
+         * if an [Exception] was thrown, otherwise [Ok].
          */
-        inline fun <T> of(function: () -> T): Result<T, Exception> {
+        inline fun <V> of(function: () -> V): Result<V, Exception> {
             return try {
                 Ok(function.invoke())
             } catch (ex: Exception) {
@@ -33,3 +33,14 @@ data class Ok<out V>(val value: V) : Result<V, Nothing>()
  * Represents a failed [Result], containing an [error].
  */
 data class Err<out E>(val error: E) : Result<Nothing, E>()
+
+/**
+ * Converts a nullable of type [V] to a [Result]. Returns [Ok] if the value is
+ * non-null, otherwise the supplied [error].
+ */
+inline infix fun <V, E> V?.toResultOr(error: () -> E): Result<V, E> {
+    return when (this) {
+        null -> Err(error())
+        else -> Ok(this)
+    }
+}

src/test/kotlin/com/github/michaelbull/result/ResultTest.kt

@@ -9,10 +9,11 @@ internal class ResultTest {
         @Test
         internal fun returnsOkIfInvocationSuccessful() {
             val callback = { "example" }
+            val result = Result.of(callback)
 
             assertEquals(
                 expected = "example",
-                actual = Result.of(callback).get()
+                actual = result.get()
             )
         }
 
@@ -20,11 +21,33 @@ internal class ResultTest {
         internal fun returnsErrIfInvocationFails() {
             val exception = IllegalArgumentException("throw me")
             val callback = { throw exception }
-            val error = Result.of(callback).getError()!!
+            val result = Result.of(callback)
 
             assertSame(
                 expected = exception,
-                actual = error
+                actual = result.getError()
+            )
+        }
+    }
+
+    internal class `toResultOr` {
+        @Test
+        internal fun returnsOkfIfNonNull() {
+            val result = "ok".toResultOr { "err" }
+
+            assertEquals(
+                expected = "ok",
+                actual = result.get()
+            )
+        }
+
+        @Test
+        internal fun returnsErrIfNull() {
+            val result = "ok".toLongOrNull().toResultOr { "err" }
+
+            assertEquals(
+                expected = "err",
+                actual = result.getError()
             )
         }
     }