Allow grouping expressions with parentheses

lib/ahem/parser.rb

       array
     when TokenKinds::LBRACE
       hash
+    when TokenKinds::LPAREN
+      eat(TokenKinds::LPAREN)
+      e = expression
+      eat(TokenKinds::RPAREN)
+      e
     else
       raise "Unexpected token #{token.type}"
     end

spec/parser_spec.rb

     expect(parse('-1;')).to eq([AST::Unary.new(:-, AST::Number.new(1.0))])
     expect(parse('!true;')).to eq([AST::Unary.new(:!, AST::Boolean.new(true))])
   end
+
+  it 'gives parenthetical expressions higher precedence' do
+    expect(parse('(1 + 2) * 3;')).to eq(
+      [
+        AST::Binary.new(
+          AST::Operators::MULTIPLY,
+          AST::Binary.new(
+            AST::Operators::ADD,
+            AST::Number.new(1.0),
+            AST::Number.new(2.0)
+          ),
+          AST::Number.new(3.0),
+        )
+      ]
+    )
+  end
 end