vm: very basic virtual machine

ast/src/main/kotlin/gay/pizza/pork/ast/FunctionLevelVisitor.kt

@@ -0,0 +1,42 @@
+package gay.pizza.pork.ast
+
+import gay.pizza.pork.ast.gen.*
+
+abstract class FunctionLevelVisitor<T> : NodeVisitor<T> {
+  override fun visitForInItem(node: ForInItem): T =
+    throw RuntimeException("Visiting ForInItem is not supported.")
+
+  override fun visitSymbol(node: Symbol): T =
+    throw RuntimeException("Visiting Symbol is not supported.")
+
+  override fun visitLetDefinition(node: LetDefinition): T {
+    topLevelUsedError("LetDefinition")
+  }
+
+  override fun visitArgumentSpec(node: ArgumentSpec): T =
+    throw RuntimeException("Visiting ArgumentSpec is not supported.")
+
+  override fun visitFunctionDefinition(node: FunctionDefinition): T {
+    topLevelUsedError("FunctionDefinition")
+  }
+
+  override fun visitImportDeclaration(node: ImportDeclaration): T {
+    topLevelUsedError("ImportDeclaration")
+  }
+
+  override fun visitImportPath(node: ImportPath): T {
+    topLevelUsedError("ImportPath")
+  }
+
+  override fun visitCompilationUnit(node: CompilationUnit): T {
+    topLevelUsedError("CompilationUnit")
+  }
+
+  override fun visitNativeFunctionDescriptor(node: NativeFunctionDescriptor): T {
+    topLevelUsedError("NativeFunctionDescriptor")
+  }
+
+  private fun topLevelUsedError(name: String): Nothing {
+    throw RuntimeException("$name cannot be visited in a FunctionVisitor.")
+  }
+}