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LATER EDIT:

  • Related questions:

Why are Scala class methods not first-class citizens?

Motivation for Scala underscore in terms of formal language theory and good style?

Usage of the _ wildcard, or placeholder syntax: Scala placeholder syntax

  • In this question I was trying to achieve a more compact syntax, and the solution involved the so-called placeholder syntax. This viewpoint is reinforced in Jason Swartz's Learning Scala, page 74:

Placeholder syntax is especially helpful when working with data structures and collections. Many of the core sorting, filtering, and other data structure methods tend to use first-class functions, and placeholder syntax reduces the amount of extra code required to call these methods.

QUESTION BODY

I'm trying out Scala's support for first-order functions, and encountered this problem, about passing methods as parameters. As far as I understood, the solution is to wrap the method with a (named or anonymous) first-order function. This worked for me:

def wrapperFn(s:String):String = s.reverse

wrapperFn can be now passed as parameter to other higher-order functions, or as value to other definitions like this one:

val otherGoodFn:(String=>String) = goodWrapperFn

So far so good. The problem came when I tried to mix both steps (avoiding the somewhat verbose use of the s parameter), and directly pass the method to a function-typed value, like this:

def errorFn:(String=>String) = String.reverse

Which throws the following error:

error: value reverse is not a member of object String
   def errorFn:(String=>String) = String.reverse
                                                ^

Which I don't understand, because this works as expected:

val s:String = "hello"
s.reverse

So apparently the method is a member of the instance, but not of the Object/Class(??) It seems that Scala handles the class and object lifetime in a very different way as Java does. So my question, now more concisely:

  • Is the error caused by some stupid mistake or is it indeed not allowed? And if not allowed, why?

Thanks in advance!

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fr_andres
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1 Answers1

3

String.reverse would be calling a static method (or a method on an object instead of a class in Scala terms).

You can do 

def errorWrapperFn:(String=>String) = _.reverse

The underscore here allows you to access the method/function parameters without having to give them a name.

Thilo
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