Question

While working my way through Cay S. Horstmann’s “Scala for the Impatient”, I noticed something interesting revealed by the first exercise in the first chapter.

1. In the Scala REPL, type 3. followed by the Tab key. What methods can be applied?

When I do this, I get the following

scala> 3.
%              &              *              +              -              /              
>              >=             >>             >>>            ^              asInstanceOf   
isInstanceOf   toByte         toChar         toDouble       toFloat        toInt          
toLong         toShort        toString       unary_+        unary_-        unary_~        
|       

But I noticed that if I hit Tab a second time, I get a slightly different list.

scala> 3.
!=             ##             %              &              *              +              
-              /                            >=             >>             >>>            ^              asInstanceOf   
equals         getClass       hashCode       isInstanceOf   toByte         toChar         
toDouble       toFloat        toInt          toLong         toShort        toString       
unary_+        unary_-        unary_~        |    

What is the REPL trying to tell me here? Is there something special about the different methods that appear the second time?

Answer 1

Hitting tab twice in the REPL raises the verbosity of the completion:

If “methodName” is among z‘s completions, and verbosity > 0 indicating
tab has been pressed twice consecutively, then we call alternativesFor
and show a list of overloaded method signatures.

The following methods from the interpreter source indicate what’s filtered for method completion when verbosity == 0 (i.e., when you’ve only hit tab once and aren’t getting the alternativesFor version):

def anyRefMethodsToShow = Set("isInstanceOf", "asInstanceOf", "toString")

def excludeEndsWith: List[String] = Nil

def excludeStartsWith: List[String] = List("<") // <byname>, <repeated>, etc.

def excludeNames: List[String] =
  (anyref.methodNames filterNot anyRefMethodsToShow) :+ "_root_"

def exclude(name: String): Boolean = (
  (name contains "$") ||
  (excludeNames contains name) ||
  (excludeEndsWith exists (name endsWith _)) ||
  (excludeStartsWith exists (name startsWith _))
)

So with one tab you’re getting the methods filtered by some rules that the interpreter developers have decided are reasonable and useful. Two tabs gives you the unfiltered version.