language: implement proper virtual machine support

vm/src/main/kotlin/gay/pizza/pork/vm/InternalMachine.kt

@@ -47,11 +47,14 @@ class InternalMachine(val world: CompiledWorld, val nativeRegistry: NativeRegist
     }
   }
 
-  fun loadLocal(id: UInt) {
+  fun localAt(id: UInt): Any {
     val localSet = locals.last()
-    val local = localSet[id]
-      ?: throw VirtualMachineException("Attempted to load local $id but it was not stored.")
-    push(local)
+    return localSet[id] ?:
+      throw VirtualMachineException("Attempted to load local $id but it was not stored.")
+  }
+
+  fun loadLocal(id: UInt) {
+    push(localAt(id))
   }
 
   fun storeLocal(id: UInt) {
@@ -108,4 +111,11 @@ class InternalMachine(val world: CompiledWorld, val nativeRegistry: NativeRegist
     exitFlag = false
     autoNextInst = true
   }
+
+  fun frame(at: UInt = inst): StackFrame? {
+    val (symbolInfo, rel) = world.symbolTable.lookup(at) ?: (null to null)
+    return if (symbolInfo != null && rel != null) {
+      StackFrame(symbolInfo, rel)
+    } else null
+  }
 }

vm/src/main/kotlin/gay/pizza/pork/vm/StackFrame.kt

@@ -0,0 +1,19 @@
+package gay.pizza.pork.vm
+
+import gay.pizza.pork.bytecode.CompiledWorld
+import gay.pizza.pork.bytecode.Op
+import gay.pizza.pork.bytecode.SymbolInfo
+
+data class StackFrame(val symbolInfo: SymbolInfo, val rel: UInt) {
+  val worldCodeOffset: UInt
+    get() = symbolInfo.offset + rel
+
+  fun opInWorld(world: CompiledWorld): Op =
+    world.code[worldCodeOffset.toInt()]
+
+  fun describeInWorld(world: CompiledWorld): String =
+    "StackFrame(${worldCodeOffset}, ${symbolInfo.slab} ${symbolInfo.symbol} + ${rel}, ${opInWorld(world)})"
+
+  override fun toString(): String =
+    "StackFrame(${worldCodeOffset}, ${symbolInfo.slab} ${symbolInfo.symbol} + ${rel})"
+}

vm/src/main/kotlin/gay/pizza/pork/vm/VirtualMachineProvider.kt

@@ -10,7 +10,7 @@ import gay.pizza.pork.frontend.World
 
 class VirtualMachineProvider(val world: World) : ExecutionContextProvider {
   override fun prepare(importLocator: ImportLocator, entryPointSymbol: Symbol, nativeRegistry: NativeRegistry): ExecutionContext {
-    val compiler = Compiler()
+    val compiler = Compiler(world)
     val slab = world.load(importLocator)
     val compilableSlab = compiler.compilableSlabs.of(slab)
     val compilableSymbol = compilableSlab.resolve(entryPointSymbol) ?:

vm/src/main/kotlin/gay/pizza/pork/vm/ops/AddOpHandler.kt

@@ -7,8 +7,8 @@ import gay.pizza.pork.vm.OpHandler
 
 object AddOpHandler : OpHandler(Opcode.Add) {
   override fun handle(machine: InternalMachine, op: Op) {
-    val left = machine.pop<Int>()
     val right = machine.pop<Int>()
+    val left = machine.pop<Int>()
     machine.push(left + right)
   }
 }

vm/src/main/kotlin/gay/pizza/pork/vm/ops/NativeOpHandler.kt

@@ -2,6 +2,7 @@ package gay.pizza.pork.vm.ops
 
 import gay.pizza.pork.bytecode.Op
 import gay.pizza.pork.bytecode.Opcode
+import gay.pizza.pork.execution.None
 import gay.pizza.pork.vm.InternalMachine
 import gay.pizza.pork.vm.OpHandler
 
@@ -9,9 +10,10 @@ object NativeOpHandler : OpHandler(Opcode.Native) {
   override fun handle(machine: InternalMachine, op: Op) {
     val argumentCount = op.args[2]
     val arguments = mutableListOf<Any>()
-    for (i in 0u until argumentCount) {
-      machine.loadLocal(i)
-      arguments.add(machine.popAnyValue())
+    var x = argumentCount
+    while (x > 0u) {
+      x--
+      arguments.add(machine.localAt(x))
     }
     val formConstant = machine.world.constantPool.read(op.args[0])
     val form = formConstant.readAsString()
@@ -22,6 +24,7 @@ object NativeOpHandler : OpHandler(Opcode.Native) {
       defs.add(machine.pop())
     }
     val function = provider.provideNativeFunction(defs)
-    function.invoke(arguments)
+    val result = function.invoke(arguments)
+    machine.push(if (result == Unit) None else result)
   }
 }

vm/src/main/kotlin/gay/pizza/pork/vm/ops/SubtractOpHandler.kt

@@ -7,8 +7,8 @@ import gay.pizza.pork.vm.OpHandler
 
 object SubtractOpHandler : OpHandler(Opcode.Subtract) {
   override fun handle(machine: InternalMachine, op: Op) {
-    val left = machine.pop<Int>()
     val right = machine.pop<Int>()
+    val left = machine.pop<Int>()
     machine.push(left - right)
   }
 }