If-Else Parsing

2 files changed, 150 insertions, 0 deletions

diff --git a/parser/parser.go b/parser/parser.go
index 310acee..e38e54c 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -67,6 +67,7 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerPrefix(tokens.MINUS, p.parsePrefixExpression)
 	p.registerPrefix(tokens.TRUE, p.parseBoolean)
 	p.registerPrefix(tokens.FALSE, p.parseBoolean)
+	p.registerPrefix(tokens.IF, p.parseIfExpression)
 
 	// Initialize the infix parse functions.
 	p.infixParseFns = make(map[tokens.TokenType]infixParseFn)
@@ -286,6 +287,49 @@ func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
 	return expression
 }
 
+func (p *Parser) parseIfExpression() ast.Expression {
+	expression := &ast.IfExpression{Token: p.curToken}
+
+	if !p.expectPeek(tokens.LPAREN) { return nil }
+
+	p.nextToken()
+	expression.Condition = p.parseExpression(LOWEST)
+
+	if !p.expectPeek(tokens.RPAREN) { return nil }
+
+	if !p.expectPeek(tokens.LBRACE) { return nil }
+
+	expression.Consequence = p.parseBlockStatement()
+
+	if p.peekTokenIs(tokens.ELSE) {
+		p.nextToken()
+
+		if !p.expectPeek(tokens.LBRACE) { return nil }
+
+		expression.Alternative = p.parseBlockStatement()
+	}
+
+	return expression
+}
+
+func (p *Parser) parseBlockStatement() *ast.BlockStatement {
+	block := &ast.BlockStatement{Token: p.curToken}
+	block.Statements = []ast.Statement{}
+
+	p.nextToken()
+
+	for !p.curTokenIs(tokens.RBRACE) && !p.curTokenIs(tokens.EOF) {
+		stmt := p.parseStatement()
+		if stmt != nil {
+			block.Statements = append(block.Statements, stmt)
+		}
+
+		p.nextToken()
+	}
+
+	return block
+}
+
 // 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
diff --git a/parser/parser_test.go b/parser/parser_test.go
index d2a8c8f..f26fa1e 100644
--- a/parser/parser_test.go
+++ b/parser/parser_test.go
@@ -546,3 +546,109 @@ func testBooleanLiteral(t *testing.T, exp ast.Expression, value bool) bool {
 
 	return true
 }
+
+// If expression tests.
+
+func TestIfExpression(t *testing.T) {
+	input := "if (x < y) { x }"
+
+	var l *lexer.Lexer = lexer.New(input)
+	var p *Parser = New(l)
+	var program *ast.Program = p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program.Statements does not contain %d statements. got=%d", 1, len(program.Statements))
+	}
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+
+	if !ok {
+		t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	exp, ok := stmt.Expression.(*ast.IfExpression)
+
+	if !ok {
+		t.Fatalf("stmt.Expression is not ast.IfExpression. got=%T", stmt.Expression)
+	}
+
+	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
+		return
+	}
+
+	if len(exp.Consequence.Statements) != 1 {
+		t.Errorf("consequence is not 1 statements. got=%d", len(exp.Consequence.Statements))
+	}
+
+	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
+
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T", exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, consequence.Expression, "x") {
+		return
+	}
+
+	if exp.Alternative != nil {
+		t.Errorf("exp.Alternative.Statements was not nil. got=%+v", exp.Alternative)
+	}
+}
+
+func TestIfElseExpression(t *testing.T) {
+	input := "if (x < y) { x } else { y }"
+
+	var l *lexer.Lexer = lexer.New(input)
+	var p *Parser = New(l)
+	var program *ast.Program = p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program.Statements does not contain %d statements. got=%d", 1, len(program.Statements))
+	}
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+
+	if !ok {
+		t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	exp, ok := stmt.Expression.(*ast.IfExpression)
+
+	if !ok {
+		t.Fatalf("stmt.Expression is not ast.IfExpression. got=%T", stmt.Expression)
+	}
+
+	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
+		return
+	}
+
+	if len(exp.Consequence.Statements) != 1 {
+		t.Errorf("consequence is not 1 statements. got=%d", len(exp.Consequence.Statements))
+	}
+
+	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
+
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T", exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, consequence.Expression, "x") {
+		return
+	}
+
+	if len(exp.Alternative.Statements) != 1 {
+		t.Errorf("consequence is not 1 statements. got=%d", len(exp.Alternative.Statements))
+	}
+
+	alternative, ok := exp.Alternative.Statements[0].(*ast.ExpressionStatement)
+
+	if !ok {
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T", exp.Alternative.Statements[0])
+	}
+
+	if !testIdentifier(t, alternative.Expression, "y") {
+		return
+	}
+}