Question

I start reading Lift framework source code, I find that there’re so many methods are defined using a name like methodName_? , is there a convention that _? has some special meaning?

def empty_? : Boolean = {}

Answer 1

The ? denotes that this is a predicate, a function returning Boolean. This convention goes back to Lisp, where ? (Scheme), p or -p (other Lisps, simulating the question mark with a “similar” letter) also denote predicates. Think of it as asking a question, “is the object empty?”

Scala will only allow mixed identifier names (containing alphanumerics and punctuation) if you separate them by _. E.g.,

scala> def iszero?(x : Int) = x == 0 
<console>:1: error: '=' expected but identifier found.
       def iszero?(x : Int) = x == 0
                 ^

doesn’t work, but

scala> def iszero_?(x : Int) = x == 0          
iszero_$qmark: (x: Int)Boolean

does.