2 files changed, 223 insertions, 0 deletions
diff --git a/parser/parser.go b/parser/parser.go
new file mode 100644
index 0000000..6e8bba6
--- /dev/null
+++ b/parser/parser.go
@@ -0,0 +1,132 @@
+package parser
+
+import (
+	"fmt"
+	"mana/ast"
+	"mana/lexer"
+	"mana/tokens"
+)
+
+// Parser represents a parser.
+type Parser struct {
+	l *lexer.Lexer
+
+	curToken  tokens.Token
+	peekToken tokens.Token
+
+	errors []string
+}
+
+// New returns a new Parser.
+func New(l *lexer.Lexer) *Parser {
+	var p *Parser = &Parser{
+		l: l,
+		errors: []string{},
+	}
+
+	// Read two tokens, so curToken and peekToken are both set.
+	p.nextToken()
+	p.nextToken()
+
+	return p
+}
+
+// nextToken advances the tokens.
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+}
+
+// ParseProgram parses a program.
+func (p *Parser) ParseProgram() *ast.Program {
+
+	// Initialize the program with an empty slice of statements.
+	var program *ast.Program = &ast.Program{}
+	program.Statements = []ast.Statement{}
+
+	// Iterate over the tokens until we reach the end of file.
+	for p.curToken.Type != tokens.EOF {
+		// Parse the statement and append it to the program.
+		var stmt ast.Statement = p.parseStatement()
+
+		if stmt != nil {
+			program.Statements = append(program.Statements, stmt)
+		}
+
+		// Advance to the next token.
+		p.nextToken()
+	}
+
+	// Return the program.
+	return program
+}
+
+// parseStatement parses a statement.
+func (p *Parser) parseStatement() ast.Statement {
+	switch p.curToken.Type {
+	case tokens.LET:
+		return p.parseLetStatement()
+	default:
+		return nil
+	}
+}
+
+// parseLetStatement parses a let statement.
+func (p *Parser) parseLetStatement() *ast.LetStatement {
+	var stmt *ast.LetStatement = &ast.LetStatement{Token: p.curToken}
+
+	if !p.expectPeek(tokens.IDENT) {
+		return nil
+	}
+
+	stmt.Name = &ast.Identifier{Token: p.curToken, Value: p.curToken.Literal}
+
+	if !p.expectPeek(tokens.ASSIGN) {
+		return nil
+	}
+
+	// TODO: We're skipping the expressions until we
+	// encounter a semicolon.
+
+	for !p.curTokenIs(tokens.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}
+
+// curTokenIs returns true if the current token is of the given type.
+func (p *Parser) curTokenIs(t tokens.TokenType) bool {
+	return p.curToken.Type == t
+}
+
+// peekTokenIs returns true if the next token is of the given type.
+func (p *Parser) peekTokenIs(t tokens.TokenType) bool {
+	return p.peekToken.Type == t
+}
+
+// expectPeek expects the next token to be of the given type.
+func (p *Parser) expectPeek(t tokens.TokenType) bool {
+	if p.peekTokenIs(t) {
+		p.nextToken()
+
+		return true
+	} else {
+		p.peekError(t)
+		
+		return false
+	}
+}
+
+// Errors returns the parser errors.
+func (p *Parser) Errors() []string {
+	return p.errors
+}
+
+// peekError returns an error message.
+func (p *Parser) peekError(t tokens.TokenType) {
+	var msg string = fmt.Sprintf("expected next token to be %s, got %s instead", t, p.peekToken.Type)
+
+	p.errors = append(p.errors, msg)
+}
+
diff --git a/parser/parser_test.go b/parser/parser_test.go
new file mode 100644
index 0000000..8f78c8b
--- /dev/null
+++ b/parser/parser_test.go
@@ -0,0 +1,91 @@
+package parser
+
+import (
+	"testing"
+	"mana/ast"
+	"mana/lexer"
+)
+
+func TestLetStatements(t *testing.T) {
+	const input string = `
+		let x = 5;
+		let y = 10;
+		let foobar = 838383;
+	`
+
+	var l *lexer.Lexer = lexer.New(input)
+	var p *Parser = New(l)
+
+	var program *ast.Program = p.ParseProgram()
+	checkParserErrors(t, p)
+
+	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))
+	}
+
+	var tests = []struct {
+		expectedIdentifier string
+	}{
+		{"x"},
+		{"y"},
+		{"foobar"},
+	}
+
+	for i, tt := range tests {
+		var stmt ast.Statement = program.Statements[i]
+
+		if !testLetStatement(t, stmt, tt.expectedIdentifier) {
+			return
+		}
+	}
+}
+
+func checkParserErrors(t *testing.T, p *Parser) {
+	var errors []string = p.Errors()
+
+	if len(errors) == 0 {
+		return
+	}
+
+	t.Errorf("parser has %d errors", len(errors))
+
+	for _, msg := range errors {
+		t.Errorf("parser error: %q", msg)
+	}
+
+	t.FailNow()
+}
+
+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
+	}
+
+	var 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("letStmt.Name.TokenLiteral() not '%s'. got=%s", name, letStmt.Name.TokenLiteral())
+
+		return false
+	}
+
+	return true
+}