Question

I am beginning Scala and I don’t understand what the s => part is/does. Could someone explain me that?

thrill.remove(s => s.length == 4)

Answer 1

It’s a way of specifying a function. A function is something that takes one or more parameters as input and gives back an output. One of the ways you can specify a function is with the => symbol.

s: String => s.length == 4 // a function that takes a String called s as input
                           // and returns a Boolean (true if the length of s is 4
                           // false otherwise)

In scala you can use functions like you use integer or strings or any other kind of basic data types.

You can assign them to variables:

scala> val f = (s: String) => s.length==4 // assigning our function to f
f: String => Boolean = <function1>

scala> f("abcd") // ...and using it
res1: Boolean = true

and you can pass them as parameters to other functions or methods:

scala> val thrill = List("foo", "bar", "baz", "bazz")
thrill: List[java.lang.String] = List(foo, bar, baz, bazz)

scala> thrill.remove(s => s.length == 4)
warning: there were 1 deprecation warnings; re-run with -deprecation for detail

res2: List[java.lang.String] = List(foo, bar, baz)

here you’re saying to the remove method: “apply this function s => s.length==4 to each element in the list, and remove all elements where that function returns true”

By the way, notice that remove is deprecated. The suggested alternative is filterNot

scala> thrill.filterNot(s => s.length == 4)
res3: List[java.lang.String] = List(foo, bar, baz)