module support

ast/ast.go

@@ -428,3 +428,25 @@ func (c *Comment) String() string {
 
 	return out.String()
 }
+
+// ImportExpression represents an `import` expression and holds the name
+// of the module being imported.
+type ImportExpression struct {
+	Token token.Token // The 'import' token
+	Name  Expression
+}
+
+func (ie *ImportExpression) TokenLiteral() string {
+	return ie.Token.Literal
+}
+func (ie *ImportExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString(ie.TokenLiteral())
+	out.WriteString("(")
+	out.WriteString(fmt.Sprintf("\"%s\"", ie.Name))
+	out.WriteString(")")
+
+	return out.String()
+}
+func (ie *ImportExpression) expressionNode() {}

builtins/builtins.go

@@ -8,43 +8,43 @@ import (
 
 // Builtins ...
 var Builtins = map[string]*object.Builtin{
-	"len":      {Name: "len", Fn: Len},
-	"input":    {Name: "input", Fn: Input},
-	"print":    {Name: "print", Fn: Print},
-	"first":    {Name: "first", Fn: First},
-	"last":     {Name: "last", Fn: Last},
-	"rest":     {Name: "rest", Fn: Rest},
-	"push":     {Name: "push", Fn: Push},
-	"pop":      {Name: "pop", Fn: Pop},
-	"exit":     {Name: "exit", Fn: Exit},
-	"assert":   {Name: "assert", Fn: Assert},
-	"bool":     {Name: "bool", Fn: Bool},
-	"int":      {Name: "int", Fn: Int},
-	"str":      {Name: "str", Fn: Str},
-	"type":     {Name: "type", Fn: TypeOf},
-	"args":     {Name: "args", Fn: Args},
-	"lower":    {Name: "lower", Fn: Lower},
-	"upper":    {Name: "upper", Fn: Upper},
-	"join":     {Name: "join", Fn: Join},
-	"split":    {Name: "split", Fn: Split},
-	"find":     {Name: "find", Fn: Find},
-	"read":     {Name: "read", Fn: Read},
-	"write":    {Name: "write", Fn: Write},
-	"ffi":      {Name: "ffi", Fn: FFI},
-	"abs":      {Name: "abs", Fn: Abs},
-	"bin":      {Name: "bin", Fn: Bin},
-	"hex":      {Name: "hex", Fn: Hex},
-	"ord":      {Name: "ord", Fn: Ord},
-	"chr":      {Name: "chr", Fn: Chr},
-	"divmod":   {Name: "divmod", Fn: Divmod},
-	"hash":     {Name: "hash", Fn: HashOf},
-	"id":       {Name: "id", Fn: IdOf},
-	"oct":      {Name: "oct", Fn: Oct},
-	"pow":      {Name: "pow", Fn: Pow},
-	"min":      {Name: "min", Fn: Min},
-	"max":      {Name: "max", Fn: Max},
-	"sorted":   {Name: "sorted", Fn: Sorted},
-	"reversed": {Name: "reversed", Fn: Reversed},
+	"len":       {Name: "len", Fn: Len},
+	"input":     {Name: "input", Fn: Input},
+	"print":     {Name: "print", Fn: Print},
+	"first":     {Name: "first", Fn: First},
+	"last":      {Name: "last", Fn: Last},
+	"rest":      {Name: "rest", Fn: Rest},
+	"push":      {Name: "push", Fn: Push},
+	"pop":       {Name: "pop", Fn: Pop},
+	"exit":      {Name: "exit", Fn: Exit},
+	"assert":    {Name: "assert", Fn: Assert},
+	"bool":      {Name: "bool", Fn: Bool},
+	"int":       {Name: "int", Fn: Int},
+	"str":       {Name: "str", Fn: Str},
+	"type":      {Name: "type", Fn: TypeOf},
+	"args":      {Name: "args", Fn: Args},
+	"lower":     {Name: "lower", Fn: Lower},
+	"upper":     {Name: "upper", Fn: Upper},
+	"join":      {Name: "join", Fn: Join},
+	"split":     {Name: "split", Fn: Split},
+	"find":      {Name: "find", Fn: Find},
+	"readfile":  {Name: "readfile", Fn: ReadFile},
+	"writefile": {Name: "writefile", Fn: WriteFile},
+	"ffi":       {Name: "ffi", Fn: FFI},
+	"abs":       {Name: "abs", Fn: Abs},
+	"bin":       {Name: "bin", Fn: Bin},
+	"hex":       {Name: "hex", Fn: Hex},
+	"ord":       {Name: "ord", Fn: Ord},
+	"chr":       {Name: "chr", Fn: Chr},
+	"divmod":    {Name: "divmod", Fn: Divmod},
+	"hash":      {Name: "hash", Fn: HashOf},
+	"id":        {Name: "id", Fn: IdOf},
+	"oct":       {Name: "oct", Fn: Oct},
+	"pow":       {Name: "pow", Fn: Pow},
+	"min":       {Name: "min", Fn: Min},
+	"max":       {Name: "max", Fn: Max},
+	"sorted":    {Name: "sorted", Fn: Sorted},
+	"reversed":  {Name: "reversed", Fn: Reversed},
 }
 
 // BuiltinsIndex ...

builtins/readfile.go

@@ -6,10 +6,10 @@ import (
 	"monkey/typing"
 )
 
-// Read ...
-func Read(args ...object.Object) object.Object {
+// ReadFile ...
+func ReadFile(args ...object.Object) object.Object {
 	if err := typing.Check(
-		"read", args,
+		"readfile", args,
 		typing.ExactArgs(1),
 		typing.WithTypes(object.STRING_OBJ),
 	); err != nil {

builtins/writefile.go

@@ -6,10 +6,10 @@ import (
 	"monkey/typing"
 )
 
-// Write ...
-func Write(args ...object.Object) object.Object {
+// WriteFile ...
+func WriteFile(args ...object.Object) object.Object {
 	if err := typing.Check(
-		"write", args,
+		"writefile", args,
 		typing.ExactArgs(2),
 		typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
 	); err != nil {

code/code.go

@@ -61,7 +61,7 @@ const (
 	OpClosure
 	OpGetFree
 	OpCurrentClosure
-	OpNoop
+	OpLoadModule
 )
 
 type Definition struct {
@@ -112,7 +112,7 @@ var definitions = map[Opcode]*Definition{
 	OpClosure:          {"OpClosure", []int{2, 1}},
 	OpGetFree:          {"OpGetFree", []int{1}},
 	OpCurrentClosure:   {"OpCurrentClosure", []int{}},
-	OpNoop:             {"OpNoop", []int{}},
+	OpLoadModule:       {"OpLoadModule", []int{}},
 }
 
 func Lookup(op byte) (*Definition, error) {

compiler/compiler.go

@@ -513,6 +513,16 @@ func (c *Compiler) Compile(node ast.Node) error {
 		afterConsequencePos := c.emit(code.OpNull)
 		c.changeOperand(jumpIfFalsePos, afterConsequencePos)
 
+	case *ast.ImportExpression:
+		c.l++
+		err := c.Compile(node.Name)
+		if err != nil {
+			return err
+		}
+		c.l--
+
+		c.emit(code.OpLoadModule)
+
 	}
 
 	return nil

compiler/compiler_test.go

@@ -1031,6 +1031,18 @@ func TestIteration(t *testing.T) {
 	runCompilerTests(t, tests)
 }
 
+func TestImportExpressions(t *testing.T) {
+	tests := []compilerTestCase2{
+		{
+			input:        `import("foo")`,
+			constants:    []interface{}{"foo"},
+			instructions: "0000 OpConstant 0\n0003 OpLoadModule\n0004 OpPop\n",
+		},
+	}
+
+	runCompilerTests2(t, tests)
+}
+
 func runCompilerTests(t *testing.T, tests []compilerTestCase) {
 	t.Helper()
 

compiler/symbol_table.go

@@ -19,7 +19,7 @@ type Symbol struct {
 type SymbolTable struct {
 	Outer *SymbolTable
 
-	store          map[string]Symbol
+	Store          map[string]Symbol
 	numDefinitions int
 
 	FreeSymbols []Symbol
@@ -34,7 +34,7 @@ func NewEnclosedSymbolTable(outer *SymbolTable) *SymbolTable {
 func NewSymbolTable() *SymbolTable {
 	s := make(map[string]Symbol)
 	free := []Symbol{}
-	return &SymbolTable{store: s, FreeSymbols: free}
+	return &SymbolTable{Store: s, FreeSymbols: free}
 }
 
 func (s *SymbolTable) Define(name string) Symbol {
@@ -45,13 +45,13 @@ func (s *SymbolTable) Define(name string) Symbol {
 		symbol.Scope = LocalScope
 	}
 
-	s.store[name] = symbol
+	s.Store[name] = symbol
 	s.numDefinitions++
 	return symbol
 }
 
 func (s *SymbolTable) Resolve(name string) (Symbol, bool) {
-	obj, ok := s.store[name]
+	obj, ok := s.Store[name]
 	if !ok && s.Outer != nil {
 		obj, ok = s.Outer.Resolve(name)
 		if !ok {
@@ -72,7 +72,7 @@ func (s *SymbolTable) Resolve(name string) (Symbol, bool) {
 
 func (s *SymbolTable) DefineBuiltin(index int, name string) Symbol {
 	symbol := Symbol{Name: name, Index: index, Scope: BuiltinScope}
-	s.store[name] = symbol
+	s.Store[name] = symbol
 	return symbol
 }
 
@@ -82,12 +82,12 @@ func (s *SymbolTable) DefineFree(original Symbol) Symbol {
 	symbol := Symbol{Name: original.Name, Index: len(s.FreeSymbols) - 1}
 	symbol.Scope = FreeScope
 
-	s.store[original.Name] = symbol
+	s.Store[original.Name] = symbol
 	return symbol
 }
 
 func (s *SymbolTable) DefineFunctionName(name string) Symbol {
 	symbol := Symbol{Name: name, Index: 0, Scope: FunctionScope}
-	s.store[name] = symbol
+	s.Store[name] = symbol
 	return symbol
 }

evaluator/evaluator.go

@@ -4,7 +4,11 @@ import (
 	"fmt"
 	"monkey/ast"
 	"monkey/builtins"
+	"monkey/lexer"
 	"monkey/object"
+	"monkey/parser"
+	"monkey/utils"
+	"os"
 	"strings"
 )
 
@@ -40,6 +44,9 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 	case *ast.WhileExpression:
 		return evalWhileExpression(node, env)
 
+	case *ast.ImportExpression:
+		return evalImportExpression(node, env)
+
 	case *ast.ReturnStatement:
 		val := Eval(node.ReturnValue, env)
 		if isError(val) {
@@ -190,6 +197,22 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 	return nil
 }
 
+func evalImportExpression(ie *ast.ImportExpression, env *object.Environment) object.Object {
+	name := Eval(ie.Name, env)
+	if isError(name) {
+		return name
+	}
+
+	if s, ok := name.(*object.String); ok {
+		attrs := EvalModule(s.Value)
+		if isError(attrs) {
+			return attrs
+		}
+		return &object.Module{Name: s.Value, Attrs: attrs}
+	}
+	return newError("ImportError: invalid import path '%s'", name)
+}
+
 func evalWhileExpression(we *ast.WhileExpression, env *object.Environment) object.Object {
 	var result object.Object
 
@@ -208,9 +231,9 @@ func evalWhileExpression(we *ast.WhileExpression, env *object.Environment) objec
 
 	if result != nil {
 		return result
-	} else {
-		return NULL
 	}
+
+	return NULL
 }
 
 func evalProgram(program *ast.Program, env *object.Environment) object.Object {
@@ -485,6 +508,29 @@ func newError(format string, a ...interface{}) *object.Error {
 	return &object.Error{Message: fmt.Sprintf(format, a...)}
 }
 
+// EvalModule evaluates the named module and returns a *object.Module objec
+func EvalModule(name string) object.Object {
+	filename := utils.FindModule(name)
+
+	b, err := os.ReadFile(filename)
+	if err != nil {
+		return newError("IOError: error reading module '%s': %s", name, err)
+	}
+
+	l := lexer.New(string(b))
+	p := parser.New(l)
+
+	module := p.ParseProgram()
+	if len(p.Errors()) != 0 {
+		return newError("ParseError: %s", p.Errors())
+	}
+
+	env := object.NewEnvironment()
+	Eval(module, env)
+
+	return env.ExportedHash()
+}
+
 func evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
 	if val, ok := env.Get(node.Value); ok {
 		return val
@@ -557,11 +603,18 @@ func evalIndexExpression(left, index object.Object) object.Object {
 		return evalArrayIndexExpression(left, index)
 	case left.Type() == object.HASH_OBJ:
 		return evalHashIndexExpression(left, index)
+	case left.Type() == object.MODULE_OBJ:
+		return EvalModuleIndexExpression(left, index)
 	default:
 		return newError("index operator not supported: %s", left.Type())
 	}
 }
 
+func EvalModuleIndexExpression(module, index object.Object) object.Object {
+	moduleObject := module.(*object.Module)
+	return evalHashIndexExpression(moduleObject.Attrs, index)
+}
+
 func evalStringIndexExpression(str, index object.Object) object.Object {
 	stringObject := str.(*object.String)
 	idx := index.(*object.Integer).Value

evaluator/evaluator_test.go

@@ -2,14 +2,51 @@ package evaluator
 
 import (
 	"errors"
+	"github.com/stretchr/testify/assert"
 	"monkey/lexer"
 	"monkey/object"
 	"monkey/parser"
+	"monkey/utils"
 	"os"
 	"path/filepath"
 	"testing"
 )
 
+func assertEvaluated(t *testing.T, expected interface{}, actual object.Object) {
+	t.Helper()
+
+	assert := assert.New(t)
+
+	switch expected.(type) {
+	case nil:
+		if _, ok := actual.(*object.Null); ok {
+			assert.True(ok)
+		} else {
+			assert.Equal(expected, actual)
+		}
+	case int:
+		if i, ok := actual.(*object.Integer); ok {
+			assert.Equal(int64(expected.(int)), i.Value)
+		} else {
+			assert.Equal(expected, actual)
+		}
+	case error:
+		if e, ok := actual.(*object.Integer); ok {
+			assert.Equal(expected.(error).Error(), e.Value)
+		} else {
+			assert.Equal(expected, actual)
+		}
+	case string:
+		if s, ok := actual.(*object.String); ok {
+			assert.Equal(expected.(string), s.Value)
+		} else {
+			assert.Equal(expected, actual)
+		}
+	default:
+		t.Fatalf("unsupported type for expected got=%T", expected)
+	}
+}
+
 func TestEvalExpressions(t *testing.T) {
 	tests := []struct {
 		input    string
@@ -814,6 +851,37 @@ func testStringObject(t *testing.T, obj object.Object, expected string) bool {
 	}
 	return true
 }
+func TestImportExpressions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{`mod := import("../testdata/mod"); mod.A`, 5},
+		{`mod := import("../testdata/mod"); mod.Sum(2, 3)`, 5},
+		{`mod := import("../testdata/mod"); mod.a`, nil},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		assertEvaluated(t, tt.expected, evaluated)
+	}
+}
+
+func TestImportSearchPaths(t *testing.T) {
+	utils.AddPath("../testdata")
+
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{`mod := import("mod"); mod.A`, 5},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		assertEvaluated(t, tt.expected, evaluated)
+	}
+}
 
 func TestExamples(t *testing.T) {
 	matches, err := filepath.Glob("../examples/*.monkey")

object/environment.go

@@ -1,5 +1,7 @@
 package object
 
+import "unicode"
+
 func NewEnclosedEnvironment(outer *Environment) *Environment {
 	env := NewEnvironment()
 	env.outer = outer
@@ -28,3 +30,18 @@ func (e *Environment) Set(name string, val Object) Object {
 	e.store[name] = val
 	return val
 }
+
+// ExportedHash returns a new Hash with the names and values of every publicly
+// exported binding in the environment. That is every binding that starts with a
+// capital letter. This is used by the module import system to wrap up the
+// evaluated module into an object.
+func (e *Environment) ExportedHash() *Hash {
+	pairs := make(map[HashKey]HashPair)
+	for k, v := range e.store {
+		if unicode.IsUpper(rune(k[0])) {
+			s := &String{Value: k}
+			pairs[s.HashKey()] = HashPair{Key: s, Value: v}
+		}
+	}
+	return &Hash{Pairs: pairs}
+}

object/module.go

@@ -0,0 +1,29 @@
+package object
+
+import "fmt"
+
+// Module is the module type used to represent a collection of variabels.
+type Module struct {
+	Name  string
+	Attrs Object
+}
+
+func (m Module) String() string {
+	return m.Inspect()
+}
+
+func (m Module) Type() ObjectType {
+	return MODULE_OBJ
+}
+
+func (m Module) Bool() bool {
+	return true
+}
+
+func (m Module) Inspect() string {
+	return fmt.Sprintf("<module '%s'>", m.Name)
+}
+
+func (m Module) Compare(other Object) int {
+	return 1
+}

object/object.go

@@ -18,6 +18,7 @@ const (
 	HASH_OBJ              = "hash"
 	COMPILED_FUNCTION_OBJ = "COMPILED_FUNCTION"
 	CLOSURE_OBJ           = "closure"
+	MODULE_OBJ            = "module"
 )
 
 // Comparable is the interface for comparing two Object and their underlying

parser/parser.go

@@ -87,6 +87,7 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerPrefix(token.LPAREN, p.parseGroupedExpression)
 	p.registerPrefix(token.IF, p.parseIfExpression)
 	p.registerPrefix(token.FUNCTION, p.parseFunctionLiteral)
+	p.registerPrefix(token.IMPORT, p.parseImportExpression)
 	p.registerPrefix(token.WHILE, p.parseWhileExpression)
 	p.registerPrefix(token.STRING, p.parseStringLiteral)
 	p.registerPrefix(token.LBRACKET, p.parseArrayLiteral)
@@ -601,3 +602,20 @@ func (p *Parser) parseBindExpression(expression ast.Expression) ast.Expression {
 
 	return be
 }
+
+func (p *Parser) parseImportExpression() ast.Expression {
+	expression := &ast.ImportExpression{Token: p.curToken}
+
+	if !p.expectPeek(token.LPAREN) {
+		return nil
+	}
+
+	p.nextToken()
+	expression.Name = p.parseExpression(LOWEST)
+
+	if !p.expectPeek(token.RPAREN) {
+		return nil
+	}
+
+	return expression
+}

parser/parser_test.go

@@ -1227,3 +1227,23 @@ func testComment(t *testing.T, s ast.Statement, expected string) bool {
 
 	return true
 }
+
+func TestParsingImportExpressions(t *testing.T) {
+	assert := assert.New(t)
+
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{`import("mod")`, `import("mod")`},
+	}
+
+	for _, tt := range tests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParserErrors(t, p)
+
+		assert.Equal(tt.expected, program.String())
+	}
+}

repl/repl.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"io"
 	"log"
-	"monkey/builtins"
 	"monkey/compiler"
 	"monkey/evaluator"
 	"monkey/lexer"
@@ -42,25 +41,6 @@ type Options struct {
 	Interactive bool
 }
 
-type VMState struct {
-	constants []object.Object
-	globals   []object.Object
-	symbols   *compiler.SymbolTable
-}
-
-func NewVMState() *VMState {
-	symbolTable := compiler.NewSymbolTable()
-	for i, builtin := range builtins.BuiltinsIndex {
-		symbolTable.DefineBuiltin(i, builtin.Name)
-	}
-
-	return &VMState{
-		constants: []object.Object{},
-		globals:   make([]object.Object, vm.GlobalsSize),
-		symbols:   symbolTable,
-	}
-}
-
 type REPL struct {
 	user string
 	args []string
@@ -101,14 +81,14 @@ func (r *REPL) Eval(f io.Reader) (env *object.Environment) {
 
 // Exec parses, compiles and executes the program given by f and returns
 // the resulting virtual machine, any errors are printed to stderr
-func (r *REPL) Exec(f io.Reader) (state *VMState) {
+func (r *REPL) Exec(f io.Reader) (state *vm.VMState) {
 	b, err := io.ReadAll(f)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "error reading source file: %s", err)
 		return
 	}
 
-	state = NewVMState()
+	state = vm.NewVMState()
 
 	l := lexer.New(string(b))
 	p := parser.New(l)
@@ -119,7 +99,7 @@ func (r *REPL) Exec(f io.Reader) (state *VMState) {
 		return
 	}
 
-	c := compiler.NewWithState(state.symbols, state.constants)
+	c := compiler.NewWithState(state.Symbols, state.Constants)
 	c.Debug = r.opts.Debug
 	err = c.Compile(program)
 	if err != nil {
@@ -128,9 +108,9 @@ func (r *REPL) Exec(f io.Reader) (state *VMState) {
 	}
 
 	code := c.Bytecode()
-	state.constants = code.Constants
+	state.Constants = code.Constants
 
-	machine := vm.NewWithGlobalState(code, state.globals)
+	machine := vm.NewWithState(code, state)
 	machine.Debug = r.opts.Debug
 	err = machine.Run()
 	if err != nil {
@@ -176,11 +156,11 @@ func (r *REPL) StartEvalLoop(in io.Reader, out io.Writer, env *object.Environmen
 }
 
 // StartExecLoop starts the REPL in a continious exec loop
-func (r *REPL) StartExecLoop(in io.Reader, out io.Writer, state *VMState) {
+func (r *REPL) StartExecLoop(in io.Reader, out io.Writer, state *vm.VMState) {
 	scanner := bufio.NewScanner(in)
 
 	if state == nil {
-		state = NewVMState()
+		state = vm.NewVMState()
 	}
 
 	for {
@@ -201,7 +181,7 @@ func (r *REPL) StartExecLoop(in io.Reader, out io.Writer, state *VMState) {
 			continue
 		}
 
-		c := compiler.NewWithState(state.symbols, state.constants)
+		c := compiler.NewWithState(state.Symbols, state.Constants)
 		c.Debug = r.opts.Debug
 		err := c.Compile(program)
 		if err != nil {
@@ -210,9 +190,9 @@ func (r *REPL) StartExecLoop(in io.Reader, out io.Writer, state *VMState) {
 		}
 
 		code := c.Bytecode()
-		state.constants = code.Constants
+		state.Constants = code.Constants
 
-		machine := vm.NewWithGlobalState(code, state.globals)
+		machine := vm.NewWithState(code, state)
 		machine.Debug = r.opts.Debug
 		err = machine.Run()
 		if err != nil {

testdata/mod.monkey

@@ -0,0 +1,3 @@
+a := 1
+A := 5
+Sum := fn(a, b) { return a + b }

token/token.go

@@ -84,6 +84,8 @@ const (
 	ELSE     = "ELSE"
 	RETURN   = "RETURN"
 	WHILE    = "WHILE"
+	// IMPORT the `import` keyword (import)
+	IMPORT = "IMPORT"
 )
 
 var keywords = map[string]TokenType{
@@ -95,6 +97,7 @@ var keywords = map[string]TokenType{
 	"else":   ELSE,
 	"return": RETURN,
 	"while":  WHILE,
+	"import": IMPORT,
 }
 
 func LookupIdent(ident string) TokenType {

utils/utils.go

@@ -0,0 +1,53 @@
+package utils
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+var SearchPaths []string
+
+func init() {
+	cwd, err := os.Getwd()
+	if err != nil {
+		log.Fatalf("error getting cwd: %s", err)
+	}
+
+	if e := os.Getenv("MONKEYPATH"); e != "" {
+		tokens := strings.Split(e, ":")
+		for _, token := range tokens {
+			AddPath(token) // ignore errors
+		}
+	} else {
+		SearchPaths = append(SearchPaths, cwd)
+	}
+}
+
+func AddPath(path string) error {
+	path = os.ExpandEnv(filepath.Clean(path))
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+	SearchPaths = append(SearchPaths, absPath)
+	return nil
+}
+
+func Exists(path string) bool {
+	_, err := os.Stat(path)
+	return err == nil
+}
+
+func FindModule(name string) string {
+	basename := fmt.Sprintf("%s.monkey", name)
+	for _, p := range SearchPaths {
+		filename := filepath.Join(p, basename)
+		if Exists(filename) {
+			return filename
+		}
+	}
+	return ""
+}

vm/vm.go

@@ -2,12 +2,17 @@ package vm
 
 import (
 	"fmt"
+	"io/ioutil"
 	"log"
 	"monkey/builtins"
 	"monkey/code"
 	"monkey/compiler"
+	"monkey/lexer"
 	"monkey/object"
+	"monkey/parser"
+	"monkey/utils"
 	"strings"
+	"unicode"
 )
 
 const StackSize = 2048
@@ -18,16 +23,88 @@ var Null = &object.Null{}
 var True = &object.Boolean{Value: true}
 var False = &object.Boolean{Value: false}
 
+// ExecModule compiles the named module and returns a *object.Module object
+func ExecModule(name string, state *VMState) (object.Object, error) {
+	filename := utils.FindModule(name)
+	if filename == "" {
+		return nil, fmt.Errorf("ImportError: no module named '%s'", name)
+	}
+
+	b, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return nil, fmt.Errorf("IOError: error reading module '%s': %s", name, err)
+	}
+
+	l := lexer.New(string(b))
+	p := parser.New(l)
+
+	module := p.ParseProgram()
+	if len(p.Errors()) != 0 {
+		return nil, fmt.Errorf("ParseError: %s", p.Errors())
+	}
+
+	c := compiler.NewWithState(state.Symbols, state.Constants)
+	err = c.Compile(module)
+	if err != nil {
+		return nil, fmt.Errorf("CompileError: %s", err)
+	}
+
+	code := c.Bytecode()
+	state.Constants = code.Constants
+
+	machine := NewWithState(code, state)
+	err = machine.Run()
+	if err != nil {
+		return nil, fmt.Errorf("RuntimeError: error loading module '%s'", err)
+	}
+
+	return state.ExportedHash(), nil
+}
+
+type VMState struct {
+	Constants []object.Object
+	Globals   []object.Object
+	Symbols   *compiler.SymbolTable
+}
+
+func NewVMState() *VMState {
+	symbolTable := compiler.NewSymbolTable()
+	for i, builtin := range builtins.BuiltinsIndex {
+		symbolTable.DefineBuiltin(i, builtin.Name)
+	}
+
+	return &VMState{
+		Constants: []object.Object{},
+		Globals:   make([]object.Object, GlobalsSize),
+		Symbols:   symbolTable,
+	}
+}
+
+// exported binding in the vm state. That is every binding that starts with a
+// capital letter. This is used by the module import system to wrap up the
+// compiled and evaulated module into an object.
+func (s *VMState) ExportedHash() *object.Hash {
+	pairs := make(map[object.HashKey]object.HashPair)
+	for name, symbol := range s.Symbols.Store {
+		if unicode.IsUpper(rune(name[0])) {
+			if symbol.Scope == compiler.GlobalScope {
+				obj := s.Globals[symbol.Index]
+				s := &object.String{Value: name}
+				pairs[s.HashKey()] = object.HashPair{Key: s, Value: obj}
+			}
+		}
+	}
+	return &object.Hash{Pairs: pairs}
+}
+
 type VM struct {
 	Debug bool
 
-	constants []object.Object
+	state *VMState
 
 	stack []object.Object
 	sp    int // Always points to the next value. Top of stack is stack[sp-1]
 
-	globals []object.Object
-
 	frames      []*Frame
 	framesIndex int
 }
@@ -40,23 +117,37 @@ func New(bytecode *compiler.Bytecode) *VM {
 	frames := make([]*Frame, MaxFrames)
 	frames[0] = mainFrame
 
+	state := NewVMState()
+	state.Constants = bytecode.Constants
+
 	return &VM{
-		constants: bytecode.Constants,
+		state: state,
 
 		stack: make([]object.Object, StackSize),
 		sp:    0,
 
-		globals: make([]object.Object, GlobalsSize),
-
 		frames:      frames,
 		framesIndex: 1,
 	}
 }
 
-func NewWithGlobalState(bytecode *compiler.Bytecode, s []object.Object) *VM {
-	vm := New(bytecode)
-	vm.globals = s
-	return vm
+func NewWithState(bytecode *compiler.Bytecode, state *VMState) *VM {
+	mainFn := &object.CompiledFunction{Instructions: bytecode.Instructions}
+	mainClosure := &object.Closure{Fn: mainFn}
+	mainFrame := NewFrame(mainClosure, 0)
+
+	frames := make([]*Frame, MaxFrames)
+	frames[0] = mainFrame
+
+	return &VM{
+		state: state,
+
+		frames:      frames,
+		framesIndex: 1,
+
+		stack: make([]object.Object, StackSize),
+		sp:    0,
+	}
 }
 
 func (vm *VM) currentFrame() *Frame {
@@ -73,6 +164,24 @@ func (vm *VM) popFrame() *Frame {
 	return vm.frames[vm.framesIndex]
 }
 
+func (vm *VM) loadModule(name object.Object) error {
+	s, ok := name.(*object.String)
+	if !ok {
+		return fmt.Errorf(
+			"TypeError: import() expected argument #1 to be `str` got `%s`",
+			name.Type(),
+		)
+	}
+
+	attrs, err := ExecModule(s.Value, vm.state)
+	if err != nil {
+		return err
+	}
+
+	module := &object.Module{Name: s.Value, Attrs: attrs}
+	return vm.push(module)
+}
+
 func (vm *VM) LastPoppedStackElem() object.Object {
 	return vm.stack[vm.sp]
 }
@@ -108,7 +217,7 @@ func (vm *VM) Run() error {
 			constIndex := code.ReadUint16(ins[ip+1:])
 			vm.currentFrame().ip += 2
 
-			err := vm.push(vm.constants[constIndex])
+			err := vm.push(vm.state.Constants[constIndex])
 			if err != nil {
 				return err
 			}
@@ -185,9 +294,9 @@ func (vm *VM) Run() error {
 
 			ref := vm.pop()
 			if immutable, ok := ref.(object.Immutable); ok {
-				vm.globals[globalIndex] = immutable.Clone()
+				vm.state.Globals[globalIndex] = immutable.Clone()
 			} else {
-				vm.globals[globalIndex] = ref
+				vm.state.Globals[globalIndex] = ref
 			}
 
 			err := vm.push(Null)
@@ -198,7 +307,7 @@ func (vm *VM) Run() error {
 		case code.OpAssignGlobal:
 			globalIndex := code.ReadUint16(ins[ip+1:])
 			vm.currentFrame().ip += 2
-			vm.globals[globalIndex] = vm.pop()
+			vm.state.Globals[globalIndex] = vm.pop()
 
 			err := vm.push(Null)
 			if err != nil {
@@ -221,7 +330,7 @@ func (vm *VM) Run() error {
 			globalIndex := code.ReadUint16(ins[ip+1:])
 			vm.currentFrame().ip += 2
 
-			err := vm.push(vm.globals[globalIndex])
+			err := vm.push(vm.state.Globals[globalIndex])
 			if err != nil {
 				return err
 			}
@@ -359,6 +468,14 @@ func (vm *VM) Run() error {
 				return err
 			}
 
+		case code.OpLoadModule:
+			name := vm.pop()
+
+			err := vm.loadModule(name)
+			if err != nil {
+				return err
+			}
+
 		}
 
 		if vm.Debug {
@@ -396,6 +513,8 @@ func (vm *VM) executeGetItem(left, index object.Object) error {
 		return vm.executeArrayGetItem(left, index)
 	case left.Type() == object.HASH_OBJ:
 		return vm.executeHashGetItem(left, index)
+	case left.Type() == object.MODULE_OBJ:
+		return vm.executeHashGetItem(left.(*object.Module).Attrs, index)
 	default:
 		return fmt.Errorf(
 			"index operator not supported: left=%s index=%s",
@@ -766,7 +885,7 @@ func (vm *VM) callBuiltin(builtin *object.Builtin, numArgs int) error {
 }
 
 func (vm *VM) pushClosure(constIndex, numFree int) error {
-	constant := vm.constants[constIndex]
+	constant := vm.state.Constants[constIndex]
 	function, ok := constant.(*object.CompiledFunction)
 	if !ok {
 		return fmt.Errorf("not a function %+v", constant)

vm/vm_test.go

@@ -7,6 +7,7 @@ import (
 	"monkey/lexer"
 	"monkey/object"
 	"monkey/parser"
+	"monkey/utils"
 	"os"
 	"path"
 	"path/filepath"
@@ -1127,3 +1128,35 @@ func BenchmarkFibonacci(b *testing.B) {
 		})
 	}
 }
+
+func TestImportExpressions(t *testing.T) {
+	tests := []vmTestCase{
+		{
+			input:    `mod := import("../testdata/mod"); mod.A`,
+			expected: 5,
+		},
+		{
+			input:    `mod := import("../testdata/mod"); mod.Sum(2, 3)`,
+			expected: 5,
+		},
+		{
+			input:    `mod := import("../testdata/mod"); mod.a`,
+			expected: nil,
+		},
+	}
+
+	runVmTests(t, tests)
+}
+
+func TestImportSearchPaths(t *testing.T) {
+	utils.AddPath("../testdata")
+
+	tests := []vmTestCase{
+		{
+			input:    `mod := import("../testdata/mod"); mod.A`,
+			expected: 5,
+		},
+	}
+
+	runVmTests(t, tests)
+}