Scala toString：是否括号？

The recommended convention is to use a parameterless method whenever
there are no parameters and the method accesses mutable state only by
reading fields of the containing object (in particular,it does not
change mutable state). This convention supports the uniform access
principle,1 which says that client code should not be affected by a
decision to implement an attribute as a field or method.

这是(非官方)Scala Style Guide(第18页)必须说的：

Scala allows the omission of parentheses on methods of arity-0 (no
arguments):

reply() 
// is the same as 
reply

However,this syntax
should only be used when the method in question has no side-effects
(purely-functional). In other words,it would be acceptable to omit
parentheses when calling queue.size,but not when calling println().
This convention mirrors the method declaration convention given above.

后者没有提到统一准入原则。

如果您的toString方法可以实现为val，则意味着该字段是不可变的。但是，如果您的类是可变的，那么toString可能并不总是产生相同的结果(例如StringBuffer)。所以在Scala中的编程意味着我们应该在两种不同的情况下使用toString()：

1)当它的值是可变的

2)有副作用

就个人而言，我认为忽略第一个更常见，更一致。在实践中，String几乎不会有副作用。所以(除非它)，总是使用toString并忽略统一访问原则(遵循风格指南)：保留括号表示副作用，而不是可变性。