commit 44d9b2c7ec9f433841fd7788073a8344f0f8ec0b
Author: tijani <tijani@tijanilawal.com>
Date:   Thu May 12 20:51:16 2022 +0000

    Monkey can now lex its basic types and keywords.
    
    git-svn-id: https://svn.tlawal.org/svn/monkey@1 f6afcba9-9ef1-4bdd-9b72-7484f5705bac

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..e3ed429
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module monkey
+
+go 1.18
diff --git a/lexer/lexer.go b/lexer/lexer.go
new file mode 100644
index 0000000..7edab73
--- /dev/null
+++ b/lexer/lexer.go
@@ -0,0 +1,102 @@
+package lexer
+
+import "monkey/token"
+
+type Lexer struct {
+	input         string
+	position      int // current position in input (the current_char)
+	read_position int // current reading position in input (after current_char)
+	current_char  byte
+}
+
+func new(input string) *Lexer {
+	l := &Lexer{input: input}
+	l.read_char()
+	return l
+}
+
+func new_token(TokenType token.TokenType, ch byte) token.Token {
+	return token.Token{Type: TokenType, Literal: string(ch)}
+}
+
+func (lexer *Lexer) read_char() {
+	if lexer.read_position >= len(lexer.input) {
+		lexer.current_char = 0
+	} else {
+		lexer.current_char = lexer.input[lexer.read_position]
+	}
+	lexer.position = lexer.read_position
+	lexer.read_position += 1
+}
+
+func (lexer *Lexer) read_identifier() string {
+	position := lexer.position
+	for is_letter(lexer.current_char) {
+		lexer.read_char()
+	}
+	return lexer.input[position:lexer.position]
+}
+
+func is_letter(ch byte) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
+}
+
+func (lexer *Lexer) skip_whitespace() {
+	for lexer.current_char == ' ' || lexer.current_char == '\t' || lexer.current_char == '\n' || lexer.current_char == '\r' {
+		lexer.read_char()
+	}
+}
+
+func (lexer *Lexer) read_number() string {
+	position := lexer.position
+	for is_digit(lexer.current_char){
+		lexer.read_char()
+	}
+	return lexer.input[position: lexer.position]
+}
+
+func is_digit(ch byte) bool{
+	return '0' <= ch && ch <= '9'
+}
+
+func (lexer *Lexer) next_token() token.Token {
+	var tok token.Token
+	lexer.skip_whitespace()
+
+	switch lexer.current_char {
+	case '=':
+		tok = new_token(token.ASSIGN, lexer.current_char)
+	case ';':
+		tok = new_token(token.SEMICOLON, lexer.current_char)
+	case '(':
+		tok = new_token(token.LPAREN, lexer.current_char)
+	case ')':
+		tok = new_token(token.RPAREN, lexer.current_char)
+	case '{':
+		tok = new_token(token.LBRACE, lexer.current_char)
+	case '}':
+		tok = new_token(token.RBRACE, lexer.current_char)
+	case ',':
+		tok = new_token(token.COMMA, lexer.current_char)
+	case '+':
+		tok = new_token(token.PLUS, lexer.current_char)
+	case 0:
+		tok.Literal = ""
+		tok.Type = token.EOF
+
+	default:
+		if is_letter(lexer.current_char) {
+			tok.Literal = lexer.read_identifier()
+			tok.Type = token.LookupIdentifier(tok.Literal)
+			return tok
+		} else if is_digit(lexer.current_char) {
+			tok.Type = token.INT
+			tok.Literal = lexer.read_number()
+			return tok
+		} else {
+			tok = new_token(token.ILLEGAL, lexer.current_char)
+		}
+	}
+	lexer.read_char()
+	return tok
+}
diff --git a/lexer/lexer_test.go b/lexer/lexer_test.go
new file mode 100644
index 0000000..910c888
--- /dev/null
+++ b/lexer/lexer_test.go
@@ -0,0 +1,73 @@
+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);
+	`
+	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.EOF, ""},
+	}
+
+	l := new(input)
+	for i, tt := range tests {
+		tok := l.next_token()
+		if tok.Type != tt.expectedType {
+			t.Fatalf("test[%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)
+		}
+	}
+}
diff --git a/token/tokens.go b/token/tokens.go
new file mode 100644
index 0000000..e9c9921
--- /dev/null
+++ b/token/tokens.go
@@ -0,0 +1,45 @@
+package token
+
+type TokenType string
+
+type Token struct {
+	Type    TokenType
+	Literal string
+}
+
+const (
+	ILLEGAL = "ILLEGAL"
+	EOF     = "EOF"
+
+	// Identifiers + Literals
+	IDENT = "IDENT"
+	INT   = "INT"
+
+	// Operators
+	ASSIGN = "="
+	PLUS   = "+"
+
+	// Delimiters
+	COMMA     = ","
+	SEMICOLON = ";"
+	LPAREN    = "("
+	RPAREN    = ")"
+	LBRACE    = "{"
+	RBRACE    = "}"
+
+	// Keywords
+	FUNCTION = "FUNCTION"
+	LET      = "LET"
+)
+
+var keywords = map[string]TokenType{
+	"fn":  FUNCTION,
+	"let": LET,
+}
+
+func LookupIdentifier(ident string) TokenType {
+	if tok, ok := keywords[ident]; ok {
+		return tok
+	}
+	return IDENT
+}