init

Readme.md

@@ -0,0 +1,15 @@
+These are my source files for working on Thorsten Ball's [Writing an interpreter in go](https://interpreterbook.com/) book. The ultimate goal of this source code is to interpret, and execute code written in a language called monkey.
+
+An example of monkey is below. Monkey is a c-like language with closures, first class functions, and variable bindings.
+
+```monkey
+
+let five = 5;
+let ten = 10;
+
+let add = fn(x, y) {
+  x + y;
+};
+return add(5,5);
+
+```

ast/ast.go

@@ -0,0 +1,46 @@
+package ast
+
+import "monkey/token"
+
+type LetStatement struct {
+	Token token.Token // the token.LET token
+	Name  *Identifier
+	Value Expression
+}
+
+func (ls *LetStatement) statementNode()       {}
+func (ls *LetStatement) TokenLiteral() string { return ls.Token.Literal }
+
+type Identifier struct {
+	Token token.Token // the token.IDENT token
+	Value string
+}
+
+func (i *Identifier) expressionNode()      {}
+func (i *Identifier) TokenLiteral() string { return i.Token.Literal }
+
+type Node interface {
+	TokenLiteral() string
+}
+
+type Statement interface {
+	Node
+	statementNode()
+}
+
+type Expression interface {
+	Node
+	expressionNode()
+}
+
+type Program struct {
+	Statements []Statement
+}
+
+func (p *Program) TokenLiteral() string {
+	if len(p.Statements) > 0 {
+		return p.Statements[0].TokenLiteral()
+	} else {
+		return ""
+	}
+}

lexer/lexer.go

@@ -0,0 +1,137 @@
+package lexer
+
+import "monkey/token"
+
+type Lexer struct {
+	input        string
+	position     int  // current position in input (points to current char)
+	readPosition int  // current reading position in input (after current char)
+	ch           byte // current char under examination
+}
+
+func New(input string) *Lexer {
+	l := &Lexer{input: input}
+	l.readChar()
+	return l
+}
+
+func (l *Lexer) NextToken() token.Token {
+	var tok token.Token
+
+	l.skipWhitespace()
+
+	switch l.ch {
+	case '=':
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.EQ, Literal: literal}
+		} else {
+			tok = newToken(token.ASSIGN, l.ch)
+		}
+	case '+':
+		tok = newToken(token.PLUS, l.ch)
+	case '-':
+		tok = newToken(token.MINUS, l.ch)
+	case '!':
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.NOT_EQ, Literal: literal}
+		} else {
+			tok = newToken(token.BANG, l.ch)
+		}
+	case '/':
+		tok = newToken(token.SLASH, l.ch)
+	case '*':
+		tok = newToken(token.ASTERISK, l.ch)
+	case '<':
+		tok = newToken(token.LT, l.ch)
+	case '>':
+		tok = newToken(token.GT, l.ch)
+	case ';':
+		tok = newToken(token.SEMICOLON, l.ch)
+	case ',':
+		tok = newToken(token.COMMA, l.ch)
+	case '{':
+		tok = newToken(token.LBRACE, l.ch)
+	case '}':
+		tok = newToken(token.RBRACE, l.ch)
+	case '(':
+		tok = newToken(token.LPAREN, l.ch)
+	case ')':
+		tok = newToken(token.RPAREN, l.ch)
+	case 0:
+		tok.Literal = ""
+		tok.Type = token.EOF
+	default:
+		if isLetter(l.ch) {
+			tok.Literal = l.readIdentifier()
+			tok.Type = token.LookupIdent(tok.Literal)
+			return tok
+		} else if isDigit(l.ch) {
+			tok.Type = token.INT
+			tok.Literal = l.readNumber()
+			return tok
+		} else {
+			tok = newToken(token.ILLEGAL, l.ch)
+		}
+	}
+
+	l.readChar()
+	return tok
+}
+
+func (l *Lexer) skipWhitespace() {
+	for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' {
+		l.readChar()
+	}
+}
+
+func (l *Lexer) readChar() {
+	if l.readPosition >= len(l.input) {
+		l.ch = 0
+	} else {
+		l.ch = l.input[l.readPosition]
+	}
+	l.position = l.readPosition
+	l.readPosition += 1
+}
+
+func (l *Lexer) peekChar() byte {
+	if l.readPosition >= len(l.input) {
+		return 0
+	} else {
+		return l.input[l.readPosition]
+	}
+}
+
+func (l *Lexer) readIdentifier() string {
+	position := l.position
+	for isLetter(l.ch) {
+		l.readChar()
+	}
+	return l.input[position:l.position]
+}
+
+func (l *Lexer) readNumber() string {
+	position := l.position
+	for isDigit(l.ch) {
+		l.readChar()
+	}
+	return l.input[position:l.position]
+}
+
+func isLetter(ch byte) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
+}
+
+func isDigit(ch byte) bool {
+	return '0' <= ch && ch <= '9'
+}
+
+func newToken(tokenType token.TokenType, ch byte) token.Token {
+	return token.Token{Type: tokenType, Literal: string(ch)}
+}

lexer/lexer_test.go

@@ -0,0 +1,126 @@
+package lexer
+
+import (
+	"testing"
+
+	"monkey/token"
+)
+
+func TestNextToken(t *testing.T) {
+	input := `let five = 5;
+let ten = 10;
+
+let add = fn(x, y) {
+  x + y;
+};
+
+let result = add(five, ten);
+!-/*5;
+5 < 10 > 5;
+
+if (5 < 10) {
+	return true;
+} else {
+	return false;
+}
+
+10 == 10;
+10 != 9;
+`
+
+	tests := []struct {
+		expectedType    token.TokenType
+		expectedLiteral string
+	}{
+		{token.LET, "let"},
+		{token.IDENT, "five"},
+		{token.ASSIGN, "="},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "ten"},
+		{token.ASSIGN, "="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "add"},
+		{token.ASSIGN, "="},
+		{token.FUNCTION, "fn"},
+		{token.LPAREN, "("},
+		{token.IDENT, "x"},
+		{token.COMMA, ","},
+		{token.IDENT, "y"},
+		{token.RPAREN, ")"},
+		{token.LBRACE, "{"},
+		{token.IDENT, "x"},
+		{token.PLUS, "+"},
+		{token.IDENT, "y"},
+		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+		{token.SEMICOLON, ";"},
+		{token.LET, "let"},
+		{token.IDENT, "result"},
+		{token.ASSIGN, "="},
+		{token.IDENT, "add"},
+		{token.LPAREN, "("},
+		{token.IDENT, "five"},
+		{token.COMMA, ","},
+		{token.IDENT, "ten"},
+		{token.RPAREN, ")"},
+		{token.SEMICOLON, ";"},
+		{token.BANG, "!"},
+		{token.MINUS, "-"},
+		{token.SLASH, "/"},
+		{token.ASTERISK, "*"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+		{token.INT, "5"},
+		{token.LT, "<"},
+		{token.INT, "10"},
+		{token.GT, ">"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+		{token.IF, "if"},
+		{token.LPAREN, "("},
+		{token.INT, "5"},
+		{token.LT, "<"},
+		{token.INT, "10"},
+		{token.RPAREN, ")"},
+		{token.LBRACE, "{"},
+		{token.RETURN, "return"},
+		{token.TRUE, "true"},
+		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+		{token.ELSE, "else"},
+		{token.LBRACE, "{"},
+		{token.RETURN, "return"},
+		{token.FALSE, "false"},
+		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+		{token.INT, "10"},
+		{token.EQ, "=="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.INT, "10"},
+		{token.NOT_EQ, "!="},
+		{token.INT, "9"},
+		{token.SEMICOLON, ";"},
+		{token.EOF, ""},
+	}
+
+	l := New(input)
+
+	for i, tt := range tests {
+		tok := l.NextToken()
+
+		if tok.Type != tt.expectedType {
+			t.Fatalf("tests[%d] - tokentype wrong. expected=%q, got=%q",
+				i, tt.expectedType, tok.Type)
+		}
+
+		if tok.Literal != tt.expectedLiteral {
+			t.Fatalf("tests[%d] - literal wrong. expected=%q, got=%q",
+				i, tt.expectedLiteral, tok.Literal)
+		}
+	}
+}

main.go

@@ -0,0 +1,21 @@
+// main.go
+
+package main
+
+import (
+	"fmt"
+	"monkey/repl"
+	"os"
+	"os/user"
+)
+
+func main() {
+	user, err := user.Current()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("Hello %s! This is the Monkey programming language!\n",
+		user.Username)
+	fmt.Printf("Feel free to type in commands\n")
+	repl.Start(os.Stdin, os.Stdout)
+}

parser/parser.go

@@ -0,0 +1,91 @@
+package parser
+
+import (
+	"monkey/ast"
+	"monkey/lexer"
+	"monkey/token"
+)
+
+type Parser struct {
+	l *lexer.Lexer
+
+	curToken  token.Token
+	peekToken token.Token
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{l: l}
+
+	// Read two tokens, so curToken and peekToken are both set
+	p.nextToken()
+	p.nextToken()
+
+	return p
+}
+
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+}
+
+func (p *Parser) ParseProgram() *ast.Program {
+	program := &ast.Program{}
+	program.Statements = []ast.Statement{}
+
+	for p.curToken.Type != token.EOF {
+		stmt := p.parseStatement()
+		if stmt != nil {
+			program.Statements = append(program.Statements, stmt)
+		}
+		p.nextToken()
+	}
+
+	return program
+}
+func (p *Parser) parseStatement() ast.Statement {
+	switch p.curToken.Type {
+	case token.LET:
+		return p.parseLetStatement()
+	default:
+		return nil
+	}
+}
+
+func (p *Parser) parseLetStatement() *ast.LetStatement {
+	stmt := &ast.LetStatement{Token: p.curToken}
+
+	if !p.expectPeek(token.IDENT) {
+		return nil
+	}
+
+	stmt.Name = &ast.Identifier{Token: p.curToken, Value: p.curToken.Literal}
+
+	if !p.expectPeek(token.ASSIGN) {
+		return nil
+	}
+
+	// TODO: We're skipping the expressions until we
+	// encounter a semicolon
+	for !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}
+
+func (p *Parser) curTokenIs(t token.TokenType) bool {
+	return p.curToken.Type == t
+}
+
+func (p *Parser) peekTokenIs(t token.TokenType) bool {
+	return p.peekToken.Type == t
+}
+
+func (p *Parser) expectPeek(t token.TokenType) bool {
+	if p.peekTokenIs(t) {
+		p.nextToken()
+		return true
+	} else {
+		return false
+	}
+}

parser/parser_test.go

@@ -0,0 +1,66 @@
+package parser
+
+import (
+	"monkey/ast"
+	"monkey/lexer"
+	"testing"
+)
+
+func TestLetStatements(t *testing.T) {
+	input := `
+let x = 5;
+let y = 10;
+let foobar = 838383;
+`
+	l := lexer.New(input)
+	p := New(l)
+
+	program := p.ParseProgram()
+	if program == nil {
+		t.Fatalf("ParseProgram() returned nil")
+	}
+	if len(program.Statements) != 3 {
+		t.Fatalf("program.Statements does not contain 3 statements. got=%d",
+			len(program.Statements))
+	}
+
+	tests := []struct {
+		expectedIdentifier string
+	}{
+		{"x"},
+		{"y"},
+		{"foobar"},
+	}
+
+	for i, tt := range tests {
+		stmt := program.Statements[i]
+		if !testLetStatement(t, stmt, tt.expectedIdentifier) {
+			return
+		}
+	}
+}
+
+func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
+	if s.TokenLiteral() != "let" {
+		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())
+		return false
+	}
+
+	letStmt, ok := s.(*ast.LetStatement)
+	if !ok {
+		t.Errorf("s not *ast.LetStatement. got=%T", s)
+		return false
+	}
+
+	if letStmt.Name.Value != name {
+		t.Errorf("letStmt.Name.Value not '%s'. got=%s", name, letStmt.Name.Value)
+		return false
+	}
+
+	if letStmt.Name.TokenLiteral() != name {
+		t.Errorf("s.Name not '%s'. got=%s", name, letStmt.Name)
+		return false
+	}
+
+	return true
+}

repl/repl.go

@@ -0,0 +1,30 @@
+package repl
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"monkey/lexer"
+	"monkey/token"
+)
+
+const PROMPT = ">> "
+
+func Start(in io.Reader, out io.Writer) {
+	scanner := bufio.NewScanner(in)
+
+	for {
+		fmt.Printf(PROMPT)
+		scanned := scanner.Scan()
+		if !scanned {
+			return
+		}
+
+		line := scanner.Text()
+		l := lexer.New(line)
+
+		for tok := l.NextToken(); tok.Type != token.EOF; tok = l.NextToken() {
+			fmt.Printf("%+v\n", tok)
+		}
+	}
+}

token/token.go

@@ -0,0 +1,66 @@
+package token
+
+type TokenType string
+
+const (
+	ILLEGAL = "ILLEGAL"
+	EOF     = "EOF"
+
+	// Identifiers + literals
+	IDENT = "IDENT" // add, foobar, x, y, ...
+	INT   = "INT"   // 1343456
+
+	// Operators
+	ASSIGN   = "="
+	PLUS     = "+"
+	MINUS    = "-"
+	BANG     = "!"
+	ASTERISK = "*"
+	SLASH    = "/"
+
+	LT = "<"
+	GT = ">"
+
+	EQ     = "=="
+	NOT_EQ = "!="
+
+	// Delimiters
+	COMMA     = ","
+	SEMICOLON = ";"
+
+	LPAREN = "("
+	RPAREN = ")"
+	LBRACE = "{"
+	RBRACE = "}"
+
+	// Keywords
+	FUNCTION = "FUNCTION"
+	LET      = "LET"
+	TRUE     = "TRUE"
+	FALSE    = "FALSE"
+	IF       = "IF"
+	ELSE     = "ELSE"
+	RETURN   = "RETURN"
+)
+
+type Token struct {
+	Type    TokenType
+	Literal string
+}
+
+var keywords = map[string]TokenType{
+	"fn":     FUNCTION,
+	"let":    LET,
+	"true":   TRUE,
+	"false":  FALSE,
+	"if":     IF,
+	"else":   ELSE,
+	"return": RETURN,
+}
+
+func LookupIdent(ident string) TokenType {
+	if tok, ok := keywords[ident]; ok {
+		return tok
+	}
+	return IDENT
+}