From 9bcaa974ca03b9f518a1e84717f79272f4d090c2 Mon Sep 17 00:00:00 2001
From: Tristan Hamilton <tristan.hamilton@workday.com>
Date: Fri, 8 May 2020 15:42:14 +0100
Subject: [PATCH] Add monad comprehensions via binding block

---
 .../com/github/michaelbull/result/Binding.kt  | 58 +++++++++++++++
 .../github/michaelbull/result/BindingTest.kt  | 74 +++++++++++++++++++
 2 files changed, 132 insertions(+)
 create mode 100644 src/main/kotlin/com/github/michaelbull/result/Binding.kt
 create mode 100644 src/test/kotlin/com/github/michaelbull/result/BindingTest.kt

diff --git a/src/main/kotlin/com/github/michaelbull/result/Binding.kt b/src/main/kotlin/com/github/michaelbull/result/Binding.kt
new file mode 100644
index 0000000..5a8a3f4
--- /dev/null
+++ b/src/main/kotlin/com/github/michaelbull/result/Binding.kt
@@ -0,0 +1,58 @@
+package com.github.michaelbull.result
+
+/**
+ * Calls the specified function [block] with [ResultBinding] as its receiver and returns its [Result].
+ *
+ * When inside a [binding] block, the [bind][ResultBinding.bind] function is accessible on any [Result]. Calling the
+ * [bind][ResultBinding.bind] function will attempt to unwrap the [Result] and locally return its [value][Ok.value]. If
+ * the [Result] is an [Err], the binding block will terminate early and return the first [error][Err.error].
+ *
+ * Example:
+ * ```
+ * fun provideX(): Result<Int, ExampleErr> { ... }
+ * fun provideY(): Result<Int, ExampleErr> { ... }
+ *
+ * val result: Result<Int, ExampleErr> = binding {
+ *   val x = provideX().bind()
+ *   val y = provideY().bind()
+ *   x + y
+ * }
+ * ```
+ *
+ * @sample com.github.michaelbull.result.bind.ResultBindingTest
+ */
+inline fun <V, E> binding(crossinline block: ResultBinding<E>.() -> V): Result<V, E> {
+    val receiver = ResultBindingImpl<E>()
+
+    return try {
+        with(receiver) { Ok(block()) }
+    } catch (ex: BindException) {
+        receiver.error
+    }
+}
+
+internal object BindException : Exception() {
+    override fun fillInStackTrace(): Throwable {
+        return this
+    }
+}
+
+interface ResultBinding<E> {
+    fun <V> Result<V, E>.bind(): V
+}
+
+@PublishedApi
+internal class ResultBindingImpl<E> : ResultBinding<E> {
+
+    lateinit var error: Err<E>
+
+    override fun <V> Result<V, E>.bind(): V {
+        return when (this) {
+            is Ok -> value
+            is Err -> {
+                this@ResultBindingImpl.error = this
+                throw BindException
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/com/github/michaelbull/result/BindingTest.kt b/src/test/kotlin/com/github/michaelbull/result/BindingTest.kt
new file mode 100644
index 0000000..939693a
--- /dev/null
+++ b/src/test/kotlin/com/github/michaelbull/result/BindingTest.kt
@@ -0,0 +1,74 @@
+package com.github.michaelbull.result
+
+import org.junit.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class BindingTest {
+
+    object BindingError
+
+    @Test
+    fun returnsOkIfAllBindsSuccessful() {
+        fun provideX(): Result<Int, BindingError> = Ok(1)
+        fun provideY(): Result<Int, BindingError> = Ok(2)
+
+        val result = binding<Int, BindingError> {
+            val x = provideX().bind()
+            val y = provideY().bind()
+            x + y
+        }
+
+        assertTrue(result is Ok)
+        assertEquals(3, result.value)
+    }
+
+    @Test
+    fun returnsOkIfAllBindsOfDifferentTypeAreSuccessful() {
+        fun provideX(): Result<String, BindingError> = Ok("1")
+        fun provideY(x: Int): Result<Int, BindingError> = Ok(x + 2)
+
+        val result = binding<Int, BindingError> {
+            val x = provideX().bind()
+            val y = provideY(x.toInt()).bind()
+            y
+        }
+
+        assertTrue(result is Ok)
+        assertEquals(3, result.value)
+    }
+
+    @Test
+    fun returnsFirstErrIfBindingFailed() {
+        fun provideX(): Result<Int, BindingError> = Ok(1)
+        fun provideY(): Result<Int, BindingError> = Err(BindingError)
+        fun provideZ(): Result<Int, BindingError> = Ok(2)
+
+        val result = binding<Int, BindingError> {
+            val x = provideX().bind()
+            val y = provideY().bind()
+            val z = provideZ().bind()
+            x + y + z
+        }
+
+        assertTrue(result is Err)
+        assertEquals(BindingError, result.error)
+    }
+
+    @Test
+    fun returnsFirstErrIfBindingsOfDifferentTypesFailed() {
+        fun provideX(): Result<Int, BindingError> = Ok(1)
+        fun provideY(): Result<String, BindingError> = Err(BindingError)
+        fun provideZ(): Result<Int, BindingError> = Ok(2)
+
+        val result = binding<Int, BindingError> {
+            val x = provideX().bind()
+            val y = provideY().bind()
+            val z = provideZ().bind()
+            x + y.toInt() + z
+        }
+
+        assertTrue(result is Err)
+        assertEquals(BindingError, result.error)
+    }
+}