What does this syntax mean? (Duration) -> String?

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I'm reading through some code that I didn't write and I've come across this instance variable within a class.

private val t: (Duration) -> String?

It's called later in the code by

val str = t(<Insert Duration here>)

What does this syntax mean in Kotlin? It's difficult to interpret the variable names, unfortunately, so I have no clue what this is doing.

kotlin syntax function
1 Answers

(Duration) -> String? is the type of the variable t. In particular here, it's a function type.

It means "function that takes a Duration as argument and returns a nullable String".

The other piece of code shows that the variable t is indeed being used as a function afterwards:

val str = t(Duration.seconds(3))

// which could also be written as
val str = t.invoke(Duration.seconds(3))
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