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What does "String => String" notation mean when putting it as the return value of a function in Scala Language?

I have this doubt for example in the following function definition (specifically in the return type "String => String"):

def myFunction(line: String): String => String = {
    _.toLowerCase()
}

Thanks in advance.

Edit: Playing a little bit with the REPL made me finally understand this.

Solution

  • A little time playing around in the Scala REPL demonstrates what's going on.

    Welcome to Scala 2.12.7 (OpenJDK 64-Bit Server VM, Java 10.0.2).
Type in expressions for evaluation. Or try :help.

scala> def myFunction(line: String): String => String = {
     |     _.toLowerCase()
     | }
myFunction: (line: String)String => String

scala> myFunction("ABCD")
res0: String => String = $$Lambda$1148/1409513883@5a0e0886

scala> res0("WxYz")
res1: String = wxyz

    Notice that the 1st passed argument, "ABCD", doesn't do anything. It's the string that gets passed to the returned function, res0, that is transformed.