Default to None in client for optional arguments

codegen/src/main/scala/caliban/codegen/ClientWriter.scala

@@ -249,9 +249,8 @@ object ClientWriter {
         )
       }
     val args = field.args match {
-      case Nil => ""
-      case list =>
-        s"(${list.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}").mkString(", ")})"
+      case Nil  => ""
+      case list => s"(${writeArgumentFields(list)})"
     }
     val argBuilder = field.args match {
       case Nil => ""
@@ -263,7 +262,14 @@ object ClientWriter {
   }
 
   def writeArgumentFields(args: List[InputValueDefinition]): String =
-    s"${args.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}").mkString(", ")}"
+    s"${args.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}${writeDefaultArgument(arg)}").mkString(", ")}"
+
+  def writeDefaultArgument(arg: InputValueDefinition): String =
+    arg.ofType match {
+      case t if t.nullable => " = None"
+      case ListType(_, _)  => " = Nil"
+      case _               => ""
+    }
 
   def writeArguments(field: FieldDefinition): String =
     if (field.args.nonEmpty) {

codegen/src/test/scala/caliban/codegen/ClientWriterSpec.scala

@@ -255,7 +255,7 @@ import caliban.client.Value._
 
 object Client {
 
-  case class CharacterInput(name: String, nicknames: List[String])
+  case class CharacterInput(name: String, nicknames: List[String] = Nil)
   object CharacterInput {
     implicit val encoder: ArgEncoder[CharacterInput] = new ArgEncoder[CharacterInput] {
       override def encode(value: CharacterInput): Value =
@@ -356,6 +356,46 @@ object Client {
     def nicknames: SelectionBuilder[Character, List[String]] = Field("nicknames", ListOf(Scalar()))
   }
 
+}
+"""
+              )
+            )
+          },
+          testM("default arguments for optional and list arguments") {
+            val schema =
+              """
+              type Query {
+                characters(
+                  first: Int!
+                  last: Int
+                  origins: [String]!
+                ): String
+              }""".stripMargin
+
+            assertM(
+              gen(schema),
+              equalTo(
+                """import caliban.client.FieldBuilder._
+import caliban.client.SelectionBuilder._
+import caliban.client._
+import caliban.client.Operations._
+
+object Client {
+
+  type Query = RootQuery
+  object Query {
+    def characters(
+      first: Int,
+      last: Option[Int] = None,
+      origins: List[Option[String]] = Nil
+    ): SelectionBuilder[RootQuery, Option[String]] =
+      Field(
+        "characters",
+        OptionOf(Scalar()),
+        arguments = List(Argument("first", first), Argument("last", last), Argument("origins", origins))
+      )
+  }
+
 }
 """
               )

core/src/main/scala/caliban/parsing/adt/Type.scala

@@ -2,7 +2,10 @@ package caliban.parsing.adt
 
 import scala.annotation.tailrec
 
-sealed trait Type
+sealed trait Type {
+  val nonNull: Boolean
+  var nullable: Boolean = !nonNull
+}
 
 object Type {
 

examples/src/main/scala/caliban/client/Client.scala

@@ -5,14 +5,10 @@ import caliban.client.FieldBuilder._
 import caliban.client.SelectionBuilder._
 import caliban.client.Operations._
 import caliban.client.Value._
-import Client.Role._
 
-/**
- * This code was auto-generated by the caliban-codegen sbt plugin
- */
 object Client {
 
-  sealed trait Origin extends Product with Serializable
+  sealed trait Origin extends scala.Product with scala.Serializable
   object Origin {
     case object BELT  extends Origin
     case object EARTH extends Origin
@@ -26,31 +22,17 @@ object Client {
     }
     implicit val encoder: ArgEncoder[Origin] = new ArgEncoder[Origin] {
       override def encode(value: Origin): Value = value match {
-        case BELT  => EnumValue("BELT")
-        case EARTH => EnumValue("EARTH")
-        case MARS  => EnumValue("MARS")
+        case Origin.BELT  => EnumValue("BELT")
+        case Origin.EARTH => EnumValue("EARTH")
+        case Origin.MARS  => EnumValue("MARS")
       }
       override def typeName: String = "Origin"
     }
   }
 
-  object Role {
-    type Captain
-    object Captain {
-      def shipName: SelectionBuilder[Captain, String] = Field("shipName", Scalar())
-    }
-    type Engineer
-    object Engineer {
-      def shipName: SelectionBuilder[Engineer, String] = Field("shipName", Scalar())
-    }
-    type Mechanic
-    object Mechanic {
-      def shipName: SelectionBuilder[Mechanic, String] = Field("shipName", Scalar())
-    }
-    type Pilot
-    object Pilot {
-      def shipName: SelectionBuilder[Pilot, String] = Field("shipName", Scalar())
-    }
+  type Engineer
+  object Engineer {
+    def shipName: SelectionBuilder[Engineer, String] = Field("shipName", Scalar())
   }
 
   type Character
@@ -79,9 +61,25 @@ object Client {
       )
   }
 
+  type Pilot
+  object Pilot {
+    def shipName: SelectionBuilder[Pilot, String] = Field("shipName", Scalar())
+  }
+
+  type Mechanic
+  object Mechanic {
+    def shipName: SelectionBuilder[Mechanic, String] = Field("shipName", Scalar())
+  }
+
+  type Captain
+  object Captain {
+    def shipName: SelectionBuilder[Captain, String] = Field("shipName", Scalar())
+  }
+
+  type Queries = RootQuery
   object Queries {
     def characters[A](
-      origin: Option[Origin]
+      origin: Option[Origin] = None
     )(innerSelection: SelectionBuilder[Character, A]): SelectionBuilder[RootQuery, List[A]] =
       Field("characters", ListOf(Obj(innerSelection)), arguments = List(Argument("origin", origin)))
     def character[A](
@@ -90,9 +88,11 @@ object Client {
       Field("character", OptionOf(Obj(innerSelection)), arguments = List(Argument("name", name)))
   }
 
+  type Mutations = RootMutation
   object Mutations {
     def deleteCharacter(name: String): SelectionBuilder[RootMutation, Boolean] =
       Field("deleteCharacter", Scalar(), arguments = List(Argument("name", name)))
   }
 
 }
+

examples/src/main/scala/caliban/client/ExampleApp.scala

@@ -1,6 +1,5 @@
 package caliban.client
 
-import caliban.client.Client.Role._
 import caliban.client.Client._
 import sttp.client._
 import sttp.client.asynchttpclient.zio.AsyncHttpClientZioBackend