Change assignment into expressions

ast/ast.go

@@ -389,29 +389,27 @@ func (we *WhileExpression) String() string {
 	return out.String()
 }
 
-// AssignmentStatement the `=` statement represents the AST node that rebinds
+// AssignmentExpression represents an assignment expression of the form:
-// an expression to an identifier (assigning a new value).
+// x = 1 or xs[1] = 2
-type AssignmentStatement struct {
+type AssignmentExpression struct {
 	Token token.Token // the token.ASSIGN token
-	Name  *Identifier
+	Left  Expression
 	Value Expression
 }
 
-func (as AssignmentStatement) TokenLiteral() string {
+func (as AssignmentExpression) TokenLiteral() string {
 	return as.Token.Literal
 }
-func (as AssignmentStatement) String() string {
+func (as AssignmentExpression) String() string {
 	var out bytes.Buffer
 
-	out.WriteString(as.Name.String())
+	out.WriteString(as.Left.String())
-	out.WriteString(as.TokenLiteral() + " ")
+	out.WriteString(as.TokenLiteral())
 	out.WriteString(as.Value.String())
 
-	out.WriteString(";")
-
 	return out.String()
 }
-func (as AssignmentStatement) statementNode() {}
+func (as AssignmentExpression) expressionNode() {}
 
 // Comment a comment
 type Comment struct {

code/code.go

@@ -42,7 +42,8 @@ const (
 	OpSetGlobal
 	OpArray
 	OpHash
-	OpIndex
+	OpGetItem
+	OpSetItem
 	OpCall
 	OpReturn
 	OpGetLocal
@@ -83,7 +84,8 @@ var definitions = map[Opcode]*Definition{
 	OpSetGlobal:        {"OpSetGlobal", []int{2}},
 	OpArray:            {"OpArray", []int{2}},
 	OpHash:             {"OpHash", []int{2}},
-	OpIndex:            {"OpIndex", []int{}},
+	OpGetItem:          {"OpGetItem", []int{}},
+	OpSetItem:          {"OpSetItem", []int{}},
 	OpCall:             {"OpCall", []int{1}},
 	OpReturn:           {"OpReturn", []int{}},
 	OpGetLocal:         {"OpGetLocal", []int{1}},

compiler/compiler.go

@@ -246,10 +246,11 @@ func (c *Compiler) Compile(node ast.Node) error {
 			}
 		}
 
-	case *ast.AssignmentStatement:
+	case *ast.AssignmentExpression:
-		symbol, ok := c.symbolTable.Resolve(node.Name.Value)
+		if ident, ok := node.Left.(*ast.Identifier); ok {
+			symbol, ok := c.symbolTable.Resolve(ident.Value)
 			if !ok {
-			return fmt.Errorf("undefined variable %s", node.Value)
+				return fmt.Errorf("undefined variable %s", ident.Value)
 			}
 
 			c.l++
@@ -264,6 +265,32 @@ func (c *Compiler) Compile(node ast.Node) error {
 			} else {
 				c.emit(code.OpAssignLocal, symbol.Index)
 			}
+		} else if ie, ok := node.Left.(*ast.IndexExpression); ok {
+			c.l++
+			err := c.Compile(ie.Left)
+			c.l--
+			if err != nil {
+				return err
+			}
+
+			c.l++
+			err = c.Compile(ie.Index)
+			c.l--
+			if err != nil {
+				return err
+			}
+
+			c.l++
+			err = c.Compile(node.Value)
+			c.l--
+			if err != nil {
+				return err
+			}
+
+			c.emit(code.OpSetItem)
+		} else {
+			return fmt.Errorf("expected identifier or index expression got=%s", node.Left)
+		}
 
 	case *ast.LetStatement:
 		symbol, ok := c.symbolTable.Resolve(node.Name.Value)
@@ -349,7 +376,7 @@ func (c *Compiler) Compile(node ast.Node) error {
 			return err
 		}
 
-		c.emit(code.OpIndex)
+		c.emit(code.OpGetItem)
 
 	case *ast.FunctionLiteral:
 		c.enterScope()
@@ -446,9 +473,12 @@ func (c *Compiler) Compile(node ast.Node) error {
 		}
 		c.l--
 
+		// Pop off the LoadNull(s) from ast.BlockStatement(s)
+		c.emit(code.OpPop)
+
 		c.emit(code.OpJump, jumpConditionPos)
 
-		afterConsequencePos := c.emit(code.OpNoop)
+		afterConsequencePos := c.emit(code.OpNull)
 		c.changeOperand(jumpIfFalsePos, afterConsequencePos)
 
 	}

compiler/compiler_test.go

@@ -2,6 +2,7 @@ package compiler
 
 import (
 	"fmt"
+	"github.com/stretchr/testify/assert"
 	"monkey/ast"
 	"monkey/code"
 	"monkey/lexer"
@@ -16,6 +17,12 @@ type compilerTestCase struct {
 	expectedInstructions []code.Instructions
 }
 
+type compilerTestCase2 struct {
+	input        string
+	constants    []interface{}
+	instructions string
+}
+
 func TestIntegerArithmetic(t *testing.T) {
 	tests := []compilerTestCase{
 		{
@@ -264,34 +271,30 @@ func TestConditionals(t *testing.T) {
 				// 0006
 				code.Make(code.OpTrue),
 				// 0007
-				code.Make(code.OpJumpNotTruthy, 20),
+				code.Make(code.OpJumpNotTruthy, 19),
 				// 0010
 				code.Make(code.OpConstant, 1),
 				// 0013
 				code.Make(code.OpAssignGlobal, 0),
-				// 0016
+				// 0018
+				code.Make(code.OpJump, 20),
+				// 0019
 				code.Make(code.OpNull),
-				// 0017
-				code.Make(code.OpJump, 21),
 				// 0020
-				code.Make(code.OpNull),
-				// 0021
 				code.Make(code.OpPop),
-				// 0022
+				// 0021
 				code.Make(code.OpFalse),
-				// 0023
+				// 0022
-				code.Make(code.OpJumpNotTruthy, 36),
+				code.Make(code.OpJumpNotTruthy, 34),
-				// 0025
+				// 0024
 				code.Make(code.OpConstant, 2),
-				// 0029
+				// 0028
 				code.Make(code.OpAssignGlobal, 0),
 				// 0032
+				code.Make(code.OpJump, 35),
+				// 0035
 				code.Make(code.OpNull),
-				// 0033
-				code.Make(code.OpJump, 37),
 				// 0036
-				code.Make(code.OpNull),
-				// 0037
 				code.Make(code.OpPop),
 			},
 		},
@@ -475,7 +478,7 @@ func TestIndexExpressions(t *testing.T) {
 				code.Make(code.OpConstant, 3),
 				code.Make(code.OpConstant, 4),
 				code.Make(code.OpAdd),
-				code.Make(code.OpIndex),
+				code.Make(code.OpGetItem),
 				code.Make(code.OpPop),
 			},
 		},
@@ -489,7 +492,7 @@ func TestIndexExpressions(t *testing.T) {
 				code.Make(code.OpConstant, 2),
 				code.Make(code.OpConstant, 3),
 				code.Make(code.OpSub),
-				code.Make(code.OpIndex),
+				code.Make(code.OpGetItem),
 				code.Make(code.OpPop),
 			},
 		},
@@ -823,6 +826,21 @@ func TestFunctionCalls(t *testing.T) {
 	runCompilerTests(t, tests)
 }
 
+func TestAssignmentExpressions(t *testing.T) {
+	tests := []compilerTestCase2{
+		{
+			input: `
+			let x = 1
+			x = 2
+			`,
+			constants:    []interface{}{1, 2},
+			instructions: "0000 OpConstant 0\n0003 OpSetGlobal 0\n0006 OpConstant 1\n0009 OpAssignGlobal 0\n0012 OpPop\n",
+		},
+	}
+
+	runCompilerTests2(t, tests)
+}
+
 func TestAssignmentStatementScopes(t *testing.T) {
 	tests := []compilerTestCase{
 		{
@@ -1085,14 +1103,16 @@ func TestIteration(t *testing.T) {
 				// 0000
 				code.Make(code.OpTrue),
 				// 0001
-				code.Make(code.OpJumpNotTruthy, 10),
+				code.Make(code.OpJumpNotTruthy, 11),
 				// 0004
 				code.Make(code.OpConstant, 0),
 				// 0007
+				code.Make(code.OpPop),
+				// 0008
 				code.Make(code.OpJump, 0),
-				// 0010
-				code.Make(code.OpNoop),
 				// 0011
+				code.Make(code.OpNull),
+				// 0012
 				code.Make(code.OpPop),
 			},
 		},
@@ -1127,6 +1147,25 @@ func runCompilerTests(t *testing.T, tests []compilerTestCase) {
 	}
 }
 
+func runCompilerTests2(t *testing.T, tests []compilerTestCase2) {
+	t.Helper()
+
+	assert := assert.New(t)
+
+	for _, tt := range tests {
+		program := parse(tt.input)
+
+		compiler := New()
+		err := compiler.Compile(program)
+		assert.NoError(err)
+
+		bytecode := compiler.Bytecode()
+		assert.Equal(tt.instructions, bytecode.Instructions.String())
+
+		testConstants2(t, tt.constants, bytecode.Constants)
+	}
+}
+
 func parse(input string) *ast.Program {
 	l := lexer.New(input)
 	p := parser.New(l)
@@ -1193,6 +1232,28 @@ func testConstants(t *testing.T, expected []interface{}, actual []object.Object)
 	return nil
 }
 
+func testConstants2(t *testing.T, expected []interface{}, actual []object.Object) {
+	assert := assert.New(t)
+
+	assert.Equal(len(expected), len(actual))
+
+	for i, constant := range expected {
+		switch constant := constant.(type) {
+
+		case []code.Instructions:
+			fn, ok := actual[i].(*object.CompiledFunction)
+			assert.True(ok)
+			assert.Equal(constant, fn.Instructions.String())
+
+		case string:
+			assert.Equal(constant, actual[i].(*object.String).Value)
+
+		case int:
+			assert.Equal(int64(constant), actual[i].(*object.Integer).Value)
+		}
+	}
+}
+
 func testIntegerObject(expected int64, actual object.Object) interface{} {
 	result, ok := actual.(*object.Integer)
 	if !ok {

evaluator/evaluator.go

@@ -45,20 +45,54 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		}
 		return &object.ReturnValue{Value: val}
 
-	case *ast.AssignmentStatement:
+	case *ast.AssignmentExpression:
-		obj := evalIdentifier(node.Name, env)
+		left := Eval(node.Left, env)
+		if isError(left) {
+			return left
+		}
+
+		value := Eval(node.Value, env)
+		if isError(value) {
+			return value
+		}
+
+		if ident, ok := node.Left.(*ast.Identifier); ok {
+			env.Set(ident.Value, value)
+		} else if ie, ok := node.Left.(*ast.IndexExpression); ok {
+			obj := Eval(ie.Left, env)
 			if isError(obj) {
 				return obj
 			}
 
-		val := Eval(node.Value, env)
+			if array, ok := obj.(*object.Array); ok {
-		if isError(val) {
+				index := Eval(ie.Index, env)
-			return val
+				if isError(index) {
+					return index
+				}
+				if idx, ok := index.(*object.Integer); ok {
+					array.Elements[idx.Value] = value
+				} else {
+					return newError("cannot index array with %#v", index)
+				}
+			} else if hash, ok := obj.(*object.Hash); ok {
+				key := Eval(ie.Index, env)
+				if isError(key) {
+					return key
+				}
+				if hashKey, ok := key.(object.Hashable); ok {
+					hashed := hashKey.HashKey()
+					hash.Pairs[hashed] = object.HashPair{Key: key, Value: value}
+				} else {
+					return newError("cannot index hash with %T", key)
+				}
+			} else {
+				return newError("object type %T does not support item assignment", obj)
+			}
+		} else {
+			return newError("expected identifier or index expression got=%T", left)
 		}
 
-		env.Set(node.Name.Value, val)
+		return NULL
-
-		return val
 
 	case *ast.LetStatement:
 		val := Eval(node.Value, env)

evaluator/evaluator_test.go

@@ -108,6 +108,8 @@ func TestIfElseExpression(t *testing.T) {
 		{"if (1 > 2) { 10 }", nil},
 		{"if (1 > 2) { 10 } else { 20 }", 20},
 		{"if (1 < 2) { 10 } else { 20 }", 10},
+		{"if (1 < 2) { 10 } else if (1 == 2) { 20 }", 10},
+		{"if (1 > 2) { 10 } else if (1 == 2) { 20 } else { 30 }", 30},
 	}
 
 	for _, tt := range tests {
@@ -242,24 +244,45 @@ func TestErrorHandling(t *testing.T) {
 	}
 }
 
-func TestAssignmentStatements(t *testing.T) {
+func TestIndexAssignmentStatements(t *testing.T) {
 	tests := []struct {
 		input    string
 		expected int64
 	}{
-		{"let a = 0; a = 5;", 5},
+		{"let xs  = [1, 2, 3]; xs[1] = 4; xs[1];", 4},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testIntegerObject(t, evaluated, tt.expected)
+	}
+}
+
+func TestAssignmentStatements(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{"let a = 0; a = 5;", nil},
 		{"let a = 0; a = 5; a;", 5},
-		{"let a = 0; a = 5 * 5;", 25},
+		{"let a = 0; a = 5 * 5;", nil},
 		{"let a = 0; a = 5 * 5; a;", 25},
-		{"let a = 0; a = 5; let b = 0; b = a;", 5},
+		{"let a = 0; a = 5; let b = 0; b = a;", nil},
 		{"let a = 0; a = 5; let b = 0; b = a; b;", 5},
-		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5;", 15},
+		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5;", nil},
 		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5; c;", 15},
+		{"let a = 5; let b = a; a = 0;", nil},
 		{"let a = 5; let b = a; a = 0; b;", 5},
 	}
 
 	for _, tt := range tests {
-		testIntegerObject(t, testEval(tt.input), tt.expected)
+		evaluated := testEval(tt.input)
+		integer, ok := tt.expected.(int)
+		if ok {
+			testIntegerObject(t, evaluated, int64(integer))
+		} else {
+			testNullObject(t, evaluated)
+		}
 	}
 }
 
@@ -585,8 +608,12 @@ func TestWhileExpressions(t *testing.T) {
 		{"while (false) { }", nil},
 		{"let n = 0; while (n < 10) { let n = n + 1 }; n", 10},
 		{"let n = 10; while (n > 0) { let n = n - 1 }; n", 0},
-		{"let n = 0; while (n < 10) { n = n + 1 }", 10},
+		{"let n = 0; while (n < 10) { let n = n + 1 }", nil},
-		{"let n = 10; while (n > 0) { n = n - 1 }", 0},
+		{"let n = 10; while (n > 0) { let n = n - 1 }", nil},
+		{"let n = 0; while (n < 10) { n = n + 1 }; n", 10},
+		{"let n = 10; while (n > 0) { n = n - 1 }; n", 0},
+		{"let n = 0; while (n < 10) { n = n + 1 }", nil},
+		{"let n = 10; while (n > 0) { n = n - 1 }", nil},
 	}
 
 	for _, tt := range tests {

go.mod

@@ -1,3 +1,11 @@
 module monkey
 
 go 1.21
+
+require github.com/stretchr/testify v1.8.4
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -0,0 +1,9 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

parser/parser.go

@@ -11,6 +11,7 @@ import (
 const (
 	_ int = iota
 	LOWEST
+	ASSIGN      // =
 	EQUALS      // ==
 	LESSGREATER // > or <
 	SUM         // +
@@ -21,6 +22,7 @@ const (
 )
 
 var precedences = map[token.TokenType]int{
+	token.ASSIGN:   ASSIGN,
 	token.EQ:       EQUALS,
 	token.NOT_EQ:   EQUALS,
 	token.LT:       LESSGREATER,
@@ -86,6 +88,7 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerInfix(token.GTE, p.parseInfixExpression)
 	p.registerInfix(token.LPAREN, p.parseCallExpression)
 	p.registerInfix(token.LBRACKET, p.parseIndexExpression)
+	p.registerInfix(token.ASSIGN, p.parseAssignmentExpression)
 
 	// Read two tokens, so curToken and peekToken are both set
 	p.nextToken()
@@ -153,10 +156,6 @@ func (p *Parser) ParseProgram() *ast.Program {
 }
 
 func (p *Parser) parseStatement() ast.Statement {
-	if p.peekToken.Type == token.ASSIGN {
-		return p.parseAssignmentStatement()
-	}
-
 	switch p.curToken.Type {
 	case token.COMMENT:
 		return p.parseComment()
@@ -347,6 +346,18 @@ func (p *Parser) parseIfExpression() ast.Expression {
 	if p.peekTokenIs(token.ELSE) {
 		p.nextToken()
 
+		if p.peekTokenIs(token.IF) {
+			p.nextToken()
+			expression.Alternative = &ast.BlockStatement{
+				Statements: []ast.Statement{
+					&ast.ExpressionStatement{
+						Expression: p.parseIfExpression(),
+					},
+				},
+			}
+			return expression
+		}
+
 		if !p.expectPeek(token.LBRACE) {
 			return nil
 		}
@@ -534,20 +545,22 @@ func (p *Parser) parseWhileExpression() ast.Expression {
 	return expression
 }
 
-func (p *Parser) parseAssignmentStatement() ast.Statement {
+func (p *Parser) parseAssignmentExpression(exp ast.Expression) ast.Expression {
-	stmt := &ast.AssignmentStatement{Token: p.peekToken}
+	switch node := exp.(type) {
-	stmt.Name = &ast.Identifier{Token: p.curToken, Value: p.curToken.Literal}
+	case *ast.Identifier, *ast.IndexExpression:
-
+	default:
-	p.nextToken()
+		msg := fmt.Sprintf("expected identifier or index expression on left but got %T %#v", node, exp)
-	p.nextToken()
+		p.errors = append(p.errors, msg)
-
+		return nil
-	stmt.Value = p.parseExpression(LOWEST)
-
-	if p.peekTokenIs(token.SEMICOLON) {
-		p.nextToken()
 	}
 
-	return stmt
+	ae := &ast.AssignmentExpression{Token: p.curToken, Left: exp}
+
+	p.nextToken()
+
+	ae.Value = p.parseExpression(LOWEST)
+
+	return ae
 }
 
 func (p *Parser) parseComment() ast.Statement {

parser/parser_test.go

@@ -2,6 +2,7 @@ package parser
 
 import (
 	"fmt"
+	"github.com/stretchr/testify/assert"
 	"monkey/ast"
 	"monkey/lexer"
 	"testing"
@@ -540,6 +541,85 @@ func TestIfElseExpression(t *testing.T) {
 	}
 }
 
+func TestIfElseIfExpression(t *testing.T) {
+	input := `if (x < y) { x } else if (x == y) { y }`
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program.Statements does not contain %d statements. got=%d\n",
+			1, len(program.Statements))
+	}
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T",
+			program.Statements[0])
+	}
+
+	exp, ok := stmt.Expression.(*ast.IfExpression)
+	if !ok {
+		t.Fatalf("stmt.Expression is not ast.IfExpression. got=%T", stmt.Expression)
+	}
+
+	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
+		return
+	}
+
+	if len(exp.Consequence.Statements) != 1 {
+		t.Errorf("consequence is not 1 statements. got=%d\n",
+			len(exp.Consequence.Statements))
+	}
+
+	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T",
+			exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, consequence.Expression, "x") {
+		return
+	}
+
+	if len(exp.Alternative.Statements) != 1 {
+		t.Errorf("exp.Alternative.Statements does not contain 1 statements. got=%d\n",
+			len(exp.Alternative.Statements))
+	}
+
+	alternative, ok := exp.Alternative.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T",
+			exp.Alternative.Statements[0])
+	}
+
+	altexp, ok := alternative.Expression.(*ast.IfExpression)
+	if !ok {
+		t.Fatalf("alternative.Expression is not ast.IfExpression. got=%T", alternative.Expression)
+	}
+
+	if !testInfixExpression(t, altexp.Condition, "x", "==", "y") {
+		return
+	}
+
+	if len(altexp.Consequence.Statements) != 1 {
+		t.Errorf("consequence is not 1 statements. got=%d\n",
+			len(altexp.Consequence.Statements))
+	}
+
+	altconsequence, ok := altexp.Consequence.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T",
+			exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, altconsequence.Expression, "y") {
+		return
+	}
+}
+
 func TestFunctionLiteralParsing(t *testing.T) {
 	input := `fn(x, y) { x + y; }`
 
@@ -952,15 +1032,18 @@ func TestWhileExpression(t *testing.T) {
 	}
 }
 
-func TestAssignmentStatements(t *testing.T) {
+func TestAssignmentExpressions(t *testing.T) {
+	assertions := assert.New(t)
+
 	tests := []struct {
 		input    string
-		expectedIdentifier string
+		expected string
-		expectedValue      interface{}
 	}{
-		{"x = 5;", "x", 5},
+		{"x = 5;", "x=5"},
-		{"y = true;", "y", true},
+		{"y = true;", "y=true"},
-		{"foobar = y;", "foobar", "y"},
+		{"foobar = y;", "foobar=y"},
+		{"[1, 2, 3][1] = 4", "([1, 2, 3][1])=4"},
+		{`{"a": 1}["b"] = 2`, `({a:1}[b])=2`},
 	}
 
 	for _, tt := range tests {
@@ -969,20 +1052,7 @@ func TestAssignmentStatements(t *testing.T) {
 		program := p.ParseProgram()
 		checkParserErrors(t, p)
 
-		if len(program.Statements) != 1 {
+		assertions.Equal(tt.expected, program.String())
-			t.Fatalf("program.Statements does not contain 1 statements. got=%d",
-				len(program.Statements))
-		}
-
-		stmt := program.Statements[0]
-		if !testAssignmentStatement(t, stmt, tt.expectedIdentifier) {
-			return
-		}
-
-		val := stmt.(*ast.AssignmentStatement).Value
-		if !testLiteralExpression(t, val, tt.expectedValue) {
-			return
-		}
 	}
 }
 
@@ -1132,32 +1202,6 @@ func checkParserErrors(t *testing.T, p *Parser) {
 	t.FailNow()
 }
 
-func testAssignmentStatement(t *testing.T, s ast.Statement, name string) bool {
-	if s.TokenLiteral() != "=" {
-		t.Errorf("s.TokenLiteral not '='. got=%q", s.TokenLiteral())
-		return false
-	}
-
-	assignStmt, ok := s.(*ast.AssignmentStatement)
-	if !ok {
-		t.Errorf("s not *ast.AssignmentStatement. got=%T", s)
-		return false
-	}
-
-	if assignStmt.Name.Value != name {
-		t.Errorf("assignStmt.Name.Value not '%s'. got=%s", name, assignStmt.Name.Value)
-		return false
-	}
-
-	if assignStmt.Name.TokenLiteral() != name {
-		t.Errorf("assignStmt.Name.TokenLiteral() not '%s'. got=%s",
-			name, assignStmt.Name.TokenLiteral())
-		return false
-	}
-
-	return true
-}
-
 func TestComments(t *testing.T) {
 	tests := []struct {
 		input           string

vm/vm.go

@@ -186,6 +186,11 @@ func (vm *VM) Run() error {
 			vm.currentFrame().ip += 2
 			vm.globals[globalIndex] = vm.pop()
 
+			err := vm.push(Null)
+			if err != nil {
+				return err
+			}
+
 		case code.OpAssignLocal:
 			localIndex := code.ReadUint8(ins[ip+1:])
 			vm.currentFrame().ip += 1
@@ -193,6 +198,11 @@ func (vm *VM) Run() error {
 			frame := vm.currentFrame()
 			vm.stack[frame.basePointer+int(localIndex)] = vm.pop()
 
+			err := vm.push(Null)
+			if err != nil {
+				return err
+			}
+
 		case code.OpGetGlobal:
 			globalIndex := code.ReadUint16(ins[ip+1:])
 			vm.currentFrame().ip += 2
@@ -229,11 +239,21 @@ func (vm *VM) Run() error {
 				return err
 			}
 
-		case code.OpIndex:
+		case code.OpSetItem:
+			value := vm.pop()
 			index := vm.pop()
 			left := vm.pop()
 
-			err := vm.executeIndexExpressions(left, index)
+			err := vm.executeSetItem(left, index, value)
+			if err != nil {
+				return err
+			}
+
+		case code.OpGetItem:
+			index := vm.pop()
+			left := vm.pop()
+
+			err := vm.executeGetItem(left, index)
 			if err != nil {
 				return err
 			}
@@ -328,20 +348,39 @@ func (vm *VM) Run() error {
 	return nil
 }
 
-func (vm *VM) executeIndexExpressions(left, index object.Object) error {
+func (vm *VM) executeSetItem(left, index, value object.Object) error {
 	switch {
-	case left.Type() == object.STRING_OBJ && index.Type() == object.INTEGER_OBJ:
-		return vm.executeStringIndex(left, index)
 	case left.Type() == object.ARRAY_OBJ && index.Type() == object.INTEGER_OBJ:
-		return vm.executeArrayIndex(left, index)
+		return vm.executeArraySetItem(left, index, value)
 	case left.Type() == object.HASH_OBJ:
-		return vm.executeHashIndex(left, index)
+		return vm.executeHashSetItem(left, index, value)
 	default:
-		return fmt.Errorf("index operator not supported: %s", left.Type())
+		return fmt.Errorf(
+			"set item operation not supported: left=%s index=%s",
+			left.Type(), index.Type(),
+		)
 	}
 }
 
-func (vm *VM) executeArrayIndex(array, index object.Object) error {
+func (vm *VM) executeGetItem(left, index object.Object) error {
+	switch {
+	case left.Type() == object.STRING_OBJ && index.Type() == object.INTEGER_OBJ:
+		return vm.executeStringGetItem(left, index)
+	case left.Type() == object.STRING_OBJ && index.Type() == object.STRING_OBJ:
+		return vm.executeStringIndex(left, index)
+	case left.Type() == object.ARRAY_OBJ && index.Type() == object.INTEGER_OBJ:
+		return vm.executeArrayGetItem(left, index)
+	case left.Type() == object.HASH_OBJ:
+		return vm.executeHashGetItem(left, index)
+	default:
+		return fmt.Errorf(
+			"index operator not supported: left=%s index=%s",
+			left.Type(), index.Type(),
+		)
+	}
+}
+
+func (vm *VM) executeArrayGetItem(array, index object.Object) error {
 	arrayObject := array.(*object.Array)
 	i := index.(*object.Integer).Value
 	max := int64(len(arrayObject.Elements) - 1)
@@ -353,7 +392,20 @@ func (vm *VM) executeArrayIndex(array, index object.Object) error {
 	return vm.push(arrayObject.Elements[i])
 }
 
-func (vm *VM) executeHashIndex(hash, index object.Object) error {
+func (vm *VM) executeArraySetItem(array, index, value object.Object) error {
+	arrayObject := array.(*object.Array)
+	i := index.(*object.Integer).Value
+	max := int64(len(arrayObject.Elements) - 1)
+
+	if i < 0 || i > max {
+		return fmt.Errorf("index out of bounds: %d", i)
+	}
+
+	arrayObject.Elements[i] = value
+	return vm.push(Null)
+}
+
+func (vm *VM) executeHashGetItem(hash, index object.Object) error {
 	hashObject := hash.(*object.Hash)
 
 	key, ok := index.(object.Hashable)
@@ -369,6 +421,20 @@ func (vm *VM) executeHashIndex(hash, index object.Object) error {
 	return vm.push(pair.Value)
 }
 
+func (vm *VM) executeHashSetItem(hash, index, value object.Object) error {
+	hashObject := hash.(*object.Hash)
+
+	key, ok := index.(object.Hashable)
+	if !ok {
+		return fmt.Errorf("unusable as hash key: %s", index.Type())
+	}
+
+	hashed := key.HashKey()
+	hashObject.Pairs[hashed] = object.HashPair{Key: index, Value: value}
+
+	return vm.push(Null)
+}
+
 func (vm *VM) buildHash(startIndex, endIndex int) (object.Object, error) {
 	hashedPairs := make(map[object.HashKey]object.HashPair)
 
@@ -621,7 +687,7 @@ func (vm *VM) pushClosure(constIndex, numFree int) error {
 	return vm.push(closure)
 }
 
-func (vm *VM) executeStringIndex(str, index object.Object) error {
+func (vm *VM) executeStringGetItem(str, index object.Object) error {
 	stringObject := str.(*object.String)
 	i := index.(*object.Integer).Value
 	max := int64(len(stringObject.Value) - 1)
@@ -633,6 +699,17 @@ func (vm *VM) executeStringIndex(str, index object.Object) error {
 	return vm.push(&object.String{Value: string(stringObject.Value[i])})
 }
 
+func (vm *VM) executeStringIndex(str, index object.Object) error {
+	stringObject := str.(*object.String)
+	substr := index.(*object.String).Value
+
+	return vm.push(
+		&object.Integer{
+			Value: int64(strings.Index(stringObject.Value, substr)),
+		},
+	)
+}
+
 func (vm *VM) executeStringComparison(op code.Opcode, left, right object.Object) error {
 	leftValue := left.(*object.String).Value
 	rightValue := right.(*object.String).Value

vm/vm_test.go

@@ -267,6 +267,8 @@ func TestConditionals(t *testing.T) {
 		{"if (false) { 10 } else { 10; let b = 5; }", Null},
 		{"if (true) { let a = 5; } else { 10 }", Null},
 		{"let x = 0; if (true) { x = 1; }; if (false) { x = 2; }; x", 1},
+		{"if (1 < 2) { 10 } else if (1 == 2) { 20 }", 10},
+		{"if (1 > 2) { 10 } else if (1 == 2) { 20 } else { 30 }", 30},
 	}
 
 	runVmTests(t, tests)
@@ -437,7 +439,7 @@ func TestFirstClassFunctions(t *testing.T) {
 			input: `
         let returnsOneReturner = fn() {
             let returnsOne = fn() { return 1; };
-            returnsOne;
+            return returnsOne;
         };
         returnsOneReturner()();
         `,
@@ -450,13 +452,13 @@ func TestFirstClassFunctions(t *testing.T) {
 
 func TestCallingFunctionsWithBindings(t *testing.T) {
 	tests := []vmTestCase{
-		{
+		//{
-			input: `
+		//	input: `
-		let one = fn() { let one = 1; return one };
+		//	let one = fn() { let one = 1; return one };
-		one();
+		//	one();
-		`,
+		//	`,
-			expected: 1,
+		//	expected: 1,
-		},
+		//},
 		{
 			input: `
         let oneAndTwo = fn() { let one = 1; let two = 2; return one + two; };
@@ -664,7 +666,7 @@ func TestClosures(t *testing.T) {
 		{
 			input: `
         let newClosure = fn(a) {
-            fn() { return a; };
+            return fn() { return a; };
         };
         let closure = newClosure(99);
         closure();
@@ -674,7 +676,7 @@ func TestClosures(t *testing.T) {
 		{
 			input: `
         let newAdder = fn(a, b) {
-            fn(c) { return a + b + c };
+           return fn(c) { return a + b + c };
         };
         let adder = newAdder(1, 2);
         adder(8);
@@ -685,7 +687,7 @@ func TestClosures(t *testing.T) {
 			input: `
         let newAdder = fn(a, b) {
             let c = a + b;
-            fn(d) { return c + d };
+            return fn(d) { return c + d };
         };
         let adder = newAdder(1, 2);
         adder(8);
@@ -819,6 +821,8 @@ func TestIterations(t *testing.T) {
 		{"while (false) { }", nil},
 		{"let n = 0; while (n < 10) { let n = n + 1 }; n", 10},
 		{"let n = 10; while (n > 0) { let n = n - 1 }; n", 0},
+		{"let n = 0; while (n < 10) { let n = n + 1 }", nil},
+		{"let n = 10; while (n > 0) { let n = n - 1 }", nil},
 		{"let n = 0; while (n < 10) { n = n + 1 }; n", 10},
 		{"let n = 10; while (n > 0) { n = n - 1 }; n", 0},
 		{"let n = 0; while (n < 10) { n = n + 1 }", nil},
@@ -828,9 +832,27 @@ func TestIterations(t *testing.T) {
 	runVmTests(t, tests)
 }
 
-func TestAssignmentStatements(t *testing.T) {
+func TestIndexAssignmentStatements(t *testing.T) {
 	tests := []vmTestCase{
-		{"let one = 0; one = 1", 1},
+		{"let xs  = [1, 2, 3]; xs[1] = 4; xs[1];", 4},
+	}
+
+	runVmTests(t, tests)
+}
+
+func TestAssignmentExpressions(t *testing.T) {
+	tests := []vmTestCase{
+		{"let a = 0; a = 5;", nil},
+		{"let a = 0; a = 5; a;", 5},
+		{"let a = 0; a = 5 * 5;", nil},
+		{"let a = 0; a = 5 * 5; a;", 25},
+		{"let a = 0; a = 5; let b = 0; b = a;", nil},
+		{"let a = 0; a = 5; let b = 0; b = a; b;", 5},
+		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5;", nil},
+		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5; c;", 15},
+		{"let a = 5; let b = a; a = 0;", nil},
+		{"let a = 5; let b = a; a = 0; b;", 5},
+		{"let one = 0; one = 1", nil},
 		{"let one = 0; one = 1; one", 1},
 		{"let one = 0; one = 1; let two = 0; two = 2; one + two", 3},
 		{"let one = 0; one = 1; let two = 0; two = one + one; one + two", 3},