Support Higher-Kinded Types in Suggestions

engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala

@@ -330,6 +330,10 @@ final class SuggestionBuilder[A: IndexedSource](val source: A) {
             .getOrElse(QualifiedName.simpleName(typeName))
           val tdef = TypeArg.Value(qualifiedTypeName)
           go(body, args :+ tdef)
+        case IR.Application.Prefix(tfun, targs, _, _, _, _) =>
+          val appFunction = go(tfun, Vector()).head
+          val appArgs     = targs.flatMap(arg => go(arg.value, Vector()))
+          args :+ TypeArg.Application(appFunction, appArgs.toVector)
         case tname: IR.Name =>
           val typeName = tname.name
           val qualifiedTypeName = resolveTypeName(bindings, typeName)
@@ -468,11 +472,20 @@ final class SuggestionBuilder[A: IndexedSource](val source: A) {
     def go(targ: TypeArg, level: Int): String =
       targ match {
         case TypeArg.Value(name) => name.toString
+        case TypeArg.Function(Vector(typeArg)) =>
+          val typeName = go(typeArg, level)
+          if (level > 0) s"($typeName)" else typeName
         case TypeArg.Function(types) =>
           val typeList = types.map(go(_, level + 1))
           if (level > 0) typeList.mkString("(", " -> ", ")")
           else typeList.mkString(" -> ")
+        case TypeArg.Application(fun, args) =>
+          val funText  = go(fun, level)
+          val argsList = args.map(go(_, level + 1)).mkString(" ")
+          val typeName = s"$funText $argsList"
+          if (level > 0) s"($typeName)" else typeName
       }
+
     go(targ, 0)
   }
 
@@ -570,6 +583,16 @@ object SuggestionBuilder {
       */
     case class Function(signature: Vector[TypeArg]) extends TypeArg
 
+    /** Function application, like `Either A B`.
+      *
+      * @param function the function type
+      * @param arguments the list of argument types
+      */
+    case class Application(
+      function: TypeArg,
+      arguments: Vector[TypeArg]
+    ) extends TypeArg
+
   }
 
   val Any: String = "Builtins.Main.Any"

engine/runtime/src/test/scala/org/enso/compiler/test/context/SuggestionBuilderTest.scala

@@ -120,6 +120,135 @@ class SuggestionBuilderTest extends CompilerTest {
       )
     }
 
+    "build method with a qualified type" in {
+      implicit val moduleContext: ModuleContext = freshModuleContext
+
+      val code =
+        """
+          |foo : Foo.Bar
+          |foo = 42""".stripMargin
+      val module = code.preprocessModule
+
+      build(code, module) shouldEqual Tree.Root(
+        Vector(
+          ModuleAtomNode,
+          Tree.Node(
+            Suggestion.Method(
+              externalId = None,
+              module     = "Unnamed.Test",
+              name       = "foo",
+              arguments = Seq(
+                Suggestion.Argument("this", "Unnamed.Test", false, false, None)
+              ),
+              selfType      = "Unnamed.Test",
+              returnType    = "Foo.Bar",
+              documentation = None
+            ),
+            Vector()
+          )
+        )
+      )
+    }
+
+    "build method with an argument" in {
+      implicit val moduleContext: ModuleContext = freshModuleContext
+
+      val code =
+        """
+          |foo : Text -> Number
+          |foo a = 42""".stripMargin
+      val module = code.preprocessModule
+
+      build(code, module) shouldEqual Tree.Root(
+        Vector(
+          ModuleAtomNode,
+          Tree.Node(
+            Suggestion.Method(
+              externalId = None,
+              module     = "Unnamed.Test",
+              name       = "foo",
+              arguments = Seq(
+                Suggestion.Argument("this", "Unnamed.Test", false, false, None),
+                Suggestion.Argument("a", "Text", false, false, None)
+              ),
+              selfType      = "Unnamed.Test",
+              returnType    = "Number",
+              documentation = None
+            ),
+            Vector()
+          )
+        )
+      )
+    }
+
+    "build method with a type containing higher kinds" in {
+      implicit val moduleContext: ModuleContext = freshModuleContext
+
+      val code =
+        """
+          |foo : Either (Vector Number) Text -> Number
+          |foo a = 42""".stripMargin
+      val module = code.preprocessModule
+
+      build(code, module) shouldEqual Tree.Root(
+        Vector(
+          ModuleAtomNode,
+          Tree.Node(
+            Suggestion.Method(
+              externalId = None,
+              module     = "Unnamed.Test",
+              name       = "foo",
+              arguments = Seq(
+                Suggestion.Argument("this", "Unnamed.Test", false, false, None),
+                Suggestion.Argument(
+                  "a",
+                  "Either (Vector Number) Text",
+                  false,
+                  false,
+                  None
+                )
+              ),
+              selfType      = "Unnamed.Test",
+              returnType    = "Number",
+              documentation = None
+            ),
+            Vector()
+          )
+        )
+      )
+    }
+
+    "build method with a type containing qualified higher kinds" in {
+      pending // issue #1711
+      implicit val moduleContext: ModuleContext = freshModuleContext
+
+      val code =
+        """
+          |foo : Foo.Bar Baz
+          |foo = 42""".stripMargin
+      val module = code.preprocessModule
+
+      build(code, module) shouldEqual Tree.Root(
+        Vector(
+          ModuleAtomNode,
+          Tree.Node(
+            Suggestion.Method(
+              externalId = None,
+              module     = "Unnamed.Test",
+              name       = "foo",
+              arguments = Seq(
+                Suggestion.Argument("this", "Unnamed.Test", false, false, None)
+              ),
+              selfType      = "Unnamed.Test",
+              returnType    = "Foo.Bar Baz",
+              documentation = None
+            ),
+            Vector()
+          )
+        )
+      )
+    }
+
     "build method with complex body" in {
       implicit val moduleContext: ModuleContext = freshModuleContext