parser: prefix expressions

ast/ast.go

@@ -132,3 +132,22 @@ func (es *ExpressionStatement) String() string {
 
 	return ""
 }
+
+type PrefixExpression struct {
+	Token    token.Token // The prefix token. Eg: !
+	Operator string
+	Right    Expression
+}
+
+func (pe *PrefixExpression) expressionNode()      {}
+func (pe *PrefixExpression) TokenLiteral() string { return pe.Token.Literal }
+func (pe *PrefixExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("(")
+	out.WriteString(pe.Operator)
+	out.WriteString(pe.Right.String())
+	out.WriteString(")")
+
+	return out.String()
+}

parser/parser.go

@@ -48,6 +48,8 @@ func New(l *lexer.Lexer) *Parser {
 	p.prefixParseFns = make(map[token.TokenType]prefixParseFn)
 	p.registerPrefix(token.IDENT, p.parseIdentifier)
 	p.registerPrefix(token.INT, p.parseIntegerLiteral)
+	p.registerPrefix(token.BANG, p.parsePrefixExpression)
+	p.registerPrefix(token.MINUS, p.parsePrefixExpression)
 
 	return p
 }
@@ -145,6 +147,7 @@ func (p *Parser) parseExpressionStatement() *ast.ExpressionStatement {
 func (p *Parser) parseExpression(precedence int) ast.Expression {
 	prefix := p.prefixParseFns[p.curToken.Type]
 	if prefix == nil {
+		p.noPrefixParseFnError(p.curToken.Type)
 		return nil
 	}
 
@@ -171,6 +174,19 @@ func (p *Parser) parseIntegerLiteral() ast.Expression {
 	return lit
 }
 
+func (p *Parser) parsePrefixExpression() ast.Expression {
+	expression := &ast.PrefixExpression{
+		Token:    p.curToken,
+		Operator: p.curToken.Literal,
+	}
+
+	p.nextToken()
+
+	expression.Right = p.parseExpression(PREFIX)
+
+	return expression
+}
+
 func (p *Parser) expectPeek(t token.TokenType) bool {
 	if p.peekTokenIs(t) {
 		p.nextToken()
@@ -193,3 +209,8 @@ func (p *Parser) peekError(t token.TokenType) {
 	msg := fmt.Sprintf("expected next token to be %s, got %s instead", t, p.peekToken.Type)
 	p.errors = append(p.errors, msg)
 }
+
+func (p *Parser) noPrefixParseFnError(t token.TokenType) {
+	msg := fmt.Sprintf("no prefix parse function for %s found", t)
+	p.errors = append(p.errors, msg)
+}

parser/parser_test.go

@@ -1,6 +1,7 @@
 package parser
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/mcapell/go-monkey/ast"
@@ -134,6 +135,46 @@ func TestIntegerLiteralExpression(t *testing.T) {
 	}
 }
 
+func TestParsingPrefixExpressions(t *testing.T) {
+	prefixTests := []struct {
+		input        string
+		operator     string
+		integerValue int64
+	}{
+		{"!5", "!", 5},
+		{"-15", "-", 15},
+	}
+
+	for _, tt := range prefixTests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParserErrors(t, p)
+
+		if len(program.Statements) != 1 {
+			t.Fatalf("program.Statements does not contain %d statements. got=%d\n", 1, len(program.Statements))
+		}
+
+		stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+		if !ok {
+			t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T\n", program.Statements[0])
+		}
+
+		exp, ok := stmt.Expression.(*ast.PrefixExpression)
+		if !ok {
+			t.Fatalf("stmt is not ast.PrefixExpression. got=%T", stmt.Expression)
+		}
+
+		if exp.Operator != tt.operator {
+			t.Fatalf("exp.Operator is not '%s'. got=%s", tt.operator, exp.Operator)
+		}
+
+		if !testIntegerLiteral(t, exp.Right, tt.integerValue) {
+			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())
@@ -159,6 +200,26 @@ func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
 	return true
 }
 
+func testIntegerLiteral(t *testing.T, il ast.Expression, value int64) bool {
+	integ, ok := il.(*ast.IntegerLiteral)
+	if !ok {
+		t.Errorf("il not *ast.IntegerLiteral. got=%T", il)
+		return false
+	}
+
+	if integ.Value != value {
+		t.Errorf("integ.Value not %d. got=%d", value, integ.Value)
+		return false
+	}
+
+	if integ.TokenLiteral() != fmt.Sprintf("%d", value) {
+		t.Errorf("integ.TokenLiteral() not %d. got=%s", value, integ.TokenLiteral())
+		return false
+	}
+
+	return true
+}
+
 func checkParserErrors(t *testing.T, p *Parser) {
 	errors := p.Errors()
 	if len(errors) == 0 {