kotlin-stdlib / kotlin / fold fold Common JVM JS Native 1.3 inline fun < R , T > Result < T > . fold ( onSuccess : ( value : T ) -> R , onFailure : ( exception : Throwable ) -> R ) : R (source) Returns the result of onSuccess for the encapsulated value if this instance represents success or the result of onFailure function for the encapsulated Throwable exception if it is failure . Note, that this function rethrows any Throwable exception thrown by onSuccess or by onFailure function.