Does anyone understand this function?

extension String {
func transform(_ transform: (String) -> String) -> String {

can anyone explain what does this function do? I am a new to swift and i have absolutely no clues. Thank you very much.

Let's rename things a bit to make this just a little easier to reason about. This is functionally the same but I've renamed the argument & added a return for clarity:

extension String {
  func transform(_ t: (String) -> String) -> String {
    return t(self)

So t is a closure that applies an arbitrary mapping of the string to another string. Being a closure, exactly what t does is specified by the caller of String.transform(..).

As an example:

let greeting = "Hello, world!"
let uppercasedExclamationGreeting = greeting.transform({
    if $0.hasSuffix("!") {
      return $0.uppercased
    } else {
      return $0
}) // "HELLO, WORLD!"

This will only uppercase your greeting if it ends with an exclamation. Drop the "!" from the end of your greeting string, and it won't be uppercased by the transform.

Also, Swift is nice in that you can leave the parentheses from the call to transform and it will still work, so most people would write:

let uppercasedExclamationGreeting = greeting.transform {

The results are the same without the parentheses.

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It is the equivalent of a mapping function.

let newText = “text”.transform { me in
    return me.uppercased()

// Or

let newText = “text”.transform { $0.uppercased() }

// newText will be “TEXT”
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