Understanding special Scala syntax

Question

I can understand this:

List("one word", "another word") unzip (_ span (_ != ' '))

But don't get what's happening here below in the span. span takes a predicate and I think the braces can be omitted as span is used inline and has only one parameter but why isn't there any _ and what is ' '.!=

List("one word", "another word") unzip (_ span ' '.!=)

Answer 1

In Scala, operators are just special-named methods. Here ' '.!= fetches but does not apply the != to the ' ' object. This method takes one parameter and returns a boolean value.

For any function f that takes a single argument, (x => f(x)) and f are equivalent expressions – so there is no need to wrap such a function into another lambda.

The span method takes a predicate, and partitions a sequence (here: String) into a prefix that satisfies the predicate, and a suffix that doesn't. Here we want to get the string up to the first space.

• In the first line, we use the predicate (_ != ' '), more clearly (c => c != ' ').
• We can now swap the arguments of the inequality “operator”, as this operation should be reflexive. We now have: (c => ' ' != c).
• We can now use the explicit method call syntax: (c => ' '.!=(c)).

• Next, we save that method in another variable:

  val f: (Char => Boolean) = ' '.!=
  ...
  (c => f(c))
  

• As stated above, f and (c => f(c)) are equivalent, therefore (_ != ' ') and ' '.!= must be equivalent too.