Compiling Expression -- compiling conditionals

code/code.go

@@ -77,6 +77,9 @@ const (
 
 	OpMinus
 	OpBang
+
+	OpJumpNotTruthy
+	OpJump
 )
 
 // Definition For debugging and testing purposes
@@ -92,19 +95,21 @@ type Definition struct {
 }
 
 var definitions = map[Opcode]*Definition{
-	OpConstant:    {"OpConstant", []int{2}},
-	OpAdd:         {"OpAdd", []int{}}, // doesn't have any operands
-	OpSub:         {"OpSub", []int{}},
-	OpMul:         {"OpMul", []int{}},
-	OpDiv:         {"OpDiv", []int{}},
-	OpPop:         {"OpPop", []int{}},
-	OpTrue:        {"OpTrue", []int{}},
-	OpFalse:       {"OpFalse", []int{}},
-	OpEqual:       {"OpEqual", []int{}},
-	OpNotEqual:    {"OpNotEqual", []int{}},
-	OpGreaterThan: {"OpGreaterThan", []int{}},
-	OpMinus:       {"OpMinus", []int{}},
-	OpBang:        {"OpBang", []int{}},
+	OpConstant:      {"OpConstant", []int{2}},
+	OpAdd:           {"OpAdd", []int{}}, // doesn't have any operands
+	OpSub:           {"OpSub", []int{}},
+	OpMul:           {"OpMul", []int{}},
+	OpDiv:           {"OpDiv", []int{}},
+	OpPop:           {"OpPop", []int{}},
+	OpTrue:          {"OpTrue", []int{}},
+	OpFalse:         {"OpFalse", []int{}},
+	OpEqual:         {"OpEqual", []int{}},
+	OpNotEqual:      {"OpNotEqual", []int{}},
+	OpGreaterThan:   {"OpGreaterThan", []int{}},
+	OpMinus:         {"OpMinus", []int{}},
+	OpBang:          {"OpBang", []int{}},
+	OpJumpNotTruthy: {"OpJumpNotTruthy", []int{2}},
+	OpJump:          {"OpJump", []int{2}},
 }
 
 func Lookup(op byte) (*Definition, error) {

compiler/compiler.go

@@ -12,15 +12,24 @@ import (
 	"github/runnignwater/monkey/object"
 )
 
+type EmittedInstruction struct {
+	Opcode   code.Opcode
+	Position int
+}
 type Compiler struct {
 	instructions code.Instructions // hold the generated bytecode
 	constants    []object.Object   // slice that serves as our constant pool
+
+	lastInstruction     EmittedInstruction
+	previousInstruction EmittedInstruction
 }
 
 func New() *Compiler {
 	return &Compiler{
-		instructions: code.Instructions{},
-		constants:    []object.Object{},
+		instructions:        code.Instructions{},
+		constants:           []object.Object{},
+		lastInstruction:     EmittedInstruction{},
+		previousInstruction: EmittedInstruction{},
 	}
 }
 
@@ -94,6 +103,52 @@ func (c *Compiler) Compile(node ast.Node) error {
 		default:
 			return fmt.Errorf("unknown operator %s", node.Operator)
 		}
+	case *ast.IfExpression:
+		err := c.Compile(node.Condition)
+		if err != nil {
+			return err
+		}
+		// Emit an 'OpJumNotTruthy` with a bogus value
+		jumpNotTruthyPos := c.emit(code.OpJumpNotTruthy, 9999)
+
+		err = c.Compile(node.Consequence)
+		if err != nil {
+			return err
+		}
+
+		if c.lastInstructionIsPop() {
+			c.removeLastPop()
+		}
+
+		if node.Alternative == nil {
+			afterConsequencePos := len(c.instructions)
+
+			c.changOperand(jumpNotTruthyPos, afterConsequencePos)
+		} else {
+			// Emit an `OpJump` with a bogus value
+			jumpPos := c.emit(code.OpJump, 9999)
+
+			afterConsequencePos := len(c.instructions)
+			c.changOperand(jumpNotTruthyPos, afterConsequencePos)
+
+			err := c.Compile(node.Alternative)
+			if err != nil {
+				return err
+			}
+			if c.lastInstructionIsPop() {
+				c.removeLastPop()
+			}
+
+			afterAlternativePos := len(c.instructions)
+			c.changOperand(jumpPos, afterAlternativePos)
+		}
+	case *ast.BlockStatement:
+		for _, s := range node.Statements {
+			err := c.Compile(s)
+			if err != nil {
+				return err
+			}
+		}
 	case *ast.Boolean:
 		if node.Value {
 			c.emit(code.OpTrue)
@@ -129,9 +184,41 @@ func (c *Compiler) addInstruction(ins []byte) int {
 func (c *Compiler) emit(op code.Opcode, operands ...int) int {
 	ins := code.Make(op, operands...)
 	pos := c.addInstruction(ins)
+
+	c.setLastInstruction(op, pos)
+
 	return pos
 }
 
+func (c *Compiler) setLastInstruction(op code.Opcode, pos int) {
+	previous := c.lastInstruction
+	last := EmittedInstruction{Opcode: op, Position: pos}
+
+	c.previousInstruction = previous
+	c.lastInstruction = last
+}
+
+func (c *Compiler) lastInstructionIsPop() bool {
+	return c.lastInstruction.Opcode == code.OpPop
+}
+
+func (c *Compiler) removeLastPop() {
+	c.instructions = c.instructions[:c.lastInstruction.Position]
+	c.lastInstruction = c.previousInstruction
+}
+
+func (c *Compiler) replaceInstruction(pos int, newInstruction []byte) {
+	for i := 0; i < len(newInstruction); i++ {
+		c.instructions[pos+i] = newInstruction[i]
+	}
+}
+func (c *Compiler) changOperand(opPos int, operand int) {
+	op := code.Opcode(c.instructions[opPos])
+	newInstruction := code.Make(op, operand)
+
+	c.replaceInstruction(opPos, newInstruction)
+}
+
 func (c *Compiler) ByteCode() *ByteCode {
 	return &ByteCode{
 		Instructions: c.instructions,

compiler/compiler_test.go

@@ -186,6 +186,53 @@ func TestBooleanExpression(t *testing.T) {
 	runCompilerTests(t, tests)
 }
 
+func TestConditionals(t *testing.T) {
+	tests := []compilerTestCase{
+		{
+			input:             "if (true) {10} ; 3333;",
+			expectedConstants: []interface{}{10, 3333},
+			expectedInstructions: []code.Instructions{
+				// 0000
+				code.Make(code.OpTrue),
+				// 0001
+				code.Make(code.OpJumpNotTruthy, 7),
+				// 0004
+				code.Make(code.OpConstant, 0),
+				// 0007
+				code.Make(code.OpPop),
+				// 0008
+				code.Make(code.OpConstant, 1),
+				// 0011
+				code.Make(code.OpPop),
+			},
+		},
+		{
+			input:             "if(true) {10} else {20}; 3333",
+			expectedConstants: []interface{}{10, 20, 3333},
+			expectedInstructions: []code.Instructions{
+				// 0000
+				code.Make(code.OpTrue),
+				// 0001
+				code.Make(code.OpJumpNotTruthy, 10),
+				// 0004
+				code.Make(code.OpConstant, 0),
+				// 0007
+				code.Make(code.OpJump, 13),
+				// 0010
+				code.Make(code.OpConstant, 1),
+				// 0013
+				code.Make(code.OpPop),
+				// 0014
+				code.Make(code.OpConstant, 2),
+				// 0017
+				code.Make(code.OpPop),
+			},
+		},
+	}
+
+	runCompilerTests(t, tests)
+}
+
 func runCompilerTests(t *testing.T, tests []compilerTestCase) {
 	t.Helper()