Allow data class with one field to be considered a value type #164

hoplite-core/src/main/kotlin/com/sksamuel/hoplite/decoder/DataClassDecoder.kt

@@ -7,6 +7,7 @@ import com.sksamuel.hoplite.ConfigResult
 import com.sksamuel.hoplite.DecoderContext
 import com.sksamuel.hoplite.Node
 import com.sksamuel.hoplite.ParameterMapper
+import com.sksamuel.hoplite.PrimitiveNode
 import com.sksamuel.hoplite.Undefined
 import com.sksamuel.hoplite.fp.ValidatedNel
 import com.sksamuel.hoplite.fp.flatMap
@@ -43,6 +44,15 @@ class DataClassDecoder : NullHandlingDecoder<Any> {
       return ConfigFailure.DataClassWithoutConstructor(klass).invalid()
     val constructor = klass.constructors.first()
 
+    // we have a special case, which is a data class with a single field with the name 'value'.
+    // we call this a "value type" and we can instantiate a value directly into this data class
+    // without needing nested config, if the node is a primitive type
+
+    // try for the value type
+    if (constructor.parameters.size == 1 && constructor.parameters[0].name == "value" && node is PrimitiveNode) {
+      return construct(type, constructor, mapOf(constructor.parameters.first() to node.value))
+    }
+
     // create a map of parameter to value. in the case of defaults, we skip the parameter completely.
     val args: ValidatedNel<ConfigFailure, List<Pair<KParameter, Any?>>> = constructor.parameters.mapNotNull { param ->
 

hoplite-yaml/src/test/kotlin/com/sksamuel/hoplite/yaml/ValueTypeDecoderTest.kt

@@ -0,0 +1,19 @@
+package com.sksamuel.hoplite.yaml
+
+import com.sksamuel.hoplite.ConfigLoader
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class ValueTypeDecoderTest : StringSpec({
+  "single value field as value type" {
+
+    // host.value doesn't exist in the config, but because it is a value type, the value of "host"
+    // will be used instead
+    data class Host(val value: String)
+    data class Server(val host: Host)
+    data class Config(val server: Server)
+
+    val config = ConfigLoader().loadConfigOrThrow<Config>("/value_type.yml")
+    config shouldBe Config(Server(Host("localhost")))
+  }
+})

hoplite-yaml/src/test/resources/value_type.yml

@@ -0,0 +1,2 @@
+server:
+  host: localhost