Parsing statements (skipping expressions).

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/parser/parser.go

@@ -0,0 +1,116 @@
+package parser
+
+import (
+	"fmt"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/ast"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/lexer"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/token"
+)
+
+type Parser struct {
+	l         *lexer.Lexer
+	errors    []string
+	curToken  token.Token
+	peekToken token.Token
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{
+		l:      l,
+		errors: []string{},
+	}
+	// TODO: figure out why this can't be done from `parseProgram`
+	p.nextToken()
+	p.nextToken()
+	return p
+}
+
+func (p *Parser) ParseProgram() *ast.Program {
+	program := &ast.Program{}
+	program.Statements = []ast.Statement{}
+	for !p.curTokenIs(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()
+	case token.RETURN:
+		return p.parseReturnStatement()
+	}
+	return nil
+}
+
+func (p *Parser) parseLetStatement() ast.Statement {
+	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: Skipping until we find the semicolon to avoid parsing the expression.
+	if !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+	return stmt
+}
+
+func (p *Parser) parseReturnStatement() ast.Statement {
+	stmt := &ast.ReturnStatement{Token: p.curToken}
+	p.nextToken()
+	// TODO: Skipping until we find the semicolon to avoid parsing the expression.
+	if !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+	return stmt
+}
+
+func (p *Parser) curTokenIs(typ token.TokenType) bool {
+	return p.curToken.Type == typ
+}
+
+func (p *Parser) peekTokenIs(typ token.TokenType) bool {
+	return p.peekToken.Type == typ
+}
+
+// NOTE: I'll leave the name as-is to avoid deviating from the book (maybe a
+// rename at the end?), but I think `nextTokenIfPeek` would be a much better
+// name for this.
+func (p *Parser) expectPeek(typ token.TokenType) bool {
+	if p.peekTokenIs(typ) {
+		p.nextToken()
+		return true
+	}
+	p.peekError(typ)
+	return false
+}
+
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+}
+
+func (p *Parser) Errors() []string {
+	return p.errors
+}
+
+func (p *Parser) peekError(typ token.TokenType) {
+	p.errors = append(
+		p.errors,
+		fmt.Sprintf(
+			"expected next token to be %q, got %q instead",
+			typ,
+			p.peekToken.Type,
+		),
+	)
+}