Make lambda return type of expect{Err} return Any

When constructing the exception message the toString method will be
implicitly called on the result of the lambda.

Takes inspiration from Kotlin's require function [1].

[1]: https://github.com/JetBrains/kotlin/blob/v1.2.0/libraries/stdlib/src/kotlin/util/Preconditions.kt#L26

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

@@ -29,7 +29,7 @@ infix fun <V, E> Result<V, E>.expect(message: String): V {
  * @param message The message to include in the [UnwrapException] if the [Result] is an [Err].
  * @throws UnwrapException if the [Result] is an [Err], with the specified [message].
  */
-infix inline fun <V, E> Result<V, E>.expect(message: () -> String): V {
+infix inline fun <V, E> Result<V, E>.expect(message: () -> Any): V {
     return when (this) {
         is Ok -> value
         is Err -> throw UnwrapException("${message()} $error")
@@ -63,7 +63,7 @@ infix fun <V, E> Result<V, E>.expectError(message: String): E {
  * @param message The message to include in the [UnwrapException] if the [Result] is [Ok].
  * @throws UnwrapException if the [Result] is [Ok], with the specified [message].
  */
-infix inline fun <V, E> Result<V, E>.expectError(message: () -> String): E {
+infix inline fun <V, E> Result<V, E>.expectError(message: () -> Any): E {
     return when (this) {
         is Ok -> throw UnwrapException("${message()} $value")
         is Err -> error

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

@@ -14,9 +14,9 @@ internal class UnwrapTest {
 
     @Test
     internal fun `unwrap should throw an UnwrapException if not ok`() {
-        val throwable = assertThrows(UnwrapException::class.java, {
+        val throwable = assertThrows(UnwrapException::class.java) {
             Err(5000).unwrap()
-        })
+        }
 
         assertThat(throwable.message, equalTo("called Result.wrap on an Err value 5000"))
     }
@@ -29,18 +29,22 @@ internal class UnwrapTest {
 
     @Test
     internal fun `expect should throw an UnwrapException with a specified message if not ok`() {
-        val throwable = assertThrows(UnwrapException::class.java, {
-            Err(1994).expect { "the year should be" }
-        })
+        val message = object {
+            override fun toString() = "the year should be"
+        }
+
+        val throwable = assertThrows(UnwrapException::class.java) {
+            Err(1994).expect { message }
+        }
 
         assertThat(throwable.message, equalTo("the year should be 1994"))
     }
 
     @Test
     internal fun `unwrapError should throw an UnwrapException if ok`() {
-        val throwable = assertThrows(UnwrapException::class.java, {
+        val throwable = assertThrows(UnwrapException::class.java) {
             Ok("example").unwrapError()
-        })
+        }
 
         assertThat(throwable.message, equalTo("called Result.unwrapError on an Ok value example"))
     }
@@ -53,9 +57,13 @@ internal class UnwrapTest {
 
     @Test
     internal fun `expectError should throw an UnwrapException with a specified message if ok`() {
-        val throwable = assertThrows(UnwrapException::class.java, {
-            Ok(2010).expectError { "the year should be" }
-        })
+        val message = object {
+            override fun toString() = "the year should be"
+        }
+
+        val throwable = assertThrows(UnwrapException::class.java) {
+            Ok(2010).expectError { message }
+        }
 
         assertThat(throwable.message, equalTo("the year should be 2010"))
     }