infix parsing and cleanup

ast/ast.go

@@ -74,6 +74,13 @@ type PrefixExpression struct {
 	Right    Expression
 }
 
+type InfixExpression struct {
+	Token    token.Token
+	Left     Expression
+	Operator string
+	Right    Expression
+}
+
 func (ls *LetStatement) statementNode() {
 }
 func (ls *LetStatement) TokenLiteral() string {
@@ -157,3 +164,19 @@ func (pe *PrefixExpression) String() string {
 
 	return out.String()
 }
+
+func (ie *InfixExpression) expressionNode() {}
+func (ie *InfixExpression) TokenLiteral() string {
+	return ie.Token.Literal
+}
+func (ie *InfixExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("(")
+	out.WriteString(ie.Left.String())
+	out.WriteString(" " + ie.Operator + " ")
+	out.WriteString(ie.Right.String())
+	out.WriteString(")")
+
+	return out.String()
+}