What is the different between below two statements

Question

def rankLangs(langs: List[String], rdd: RDD[WikipediaArticle]): List[(String, Int)] = langs.map(lang => (lang, occurrencesOfLang(lang ,rdd))).sortBy(-_._2)

def rankLangs(langs: List[String], rdd: RDD[WikipediaArticle]): List[(String, Int)] = langs.map(_ => (_, occurrencesOfLang(_, rdd))).sortBy(-_._2).reverse

The IDE says no error for the first one, but second one is marked as error. Why I could not replace lang as _?

Answer 1

This is since in the second you're using '_' inside the definition of the passing anonymous function.

Thus,

langs.map(_ => (_ , occurrencesOfLang(_,rdd)))

will expand to something like

langs.map(x$1 => (x$2:Any) => (x$2, occurrencesOfLang(x$2,rdd))

thus will return List[String=> (Any) => (String,Int)] instead of List[(String,Int)]

This is because it is equivalent to writing

langs.map(x => (_:Any) => (_, occurrencesOfLang(_,rdd))

As explained here: What are the rules to govern underscore to define anonymous function?

A much cleaner explanation with a suitable example can be found here: scala passing function with underscore produces a function not a value