An operator that combines scan(...) and transform { ... } for Kotlin flows

ScanTransform.kt

/**
 * Applies [transform] function to each value of the given flow.
 *
 * The receiver transforms is [FlowCollector] and thus [transform] is a flexible block which can
 * transform the given element, skit it or emit multiple elements.
 *
 * Additionally, this transform is stateful : each time an item is processed, it is transformed and
 * passed around for the next invocations of [transform].
 *
 * @param T the type of the elements of the given flow.
 * @param R the type of the elements of the produced flow.
 * @param S the type of the passed state.
 */
inline fun <T, R, S> Flow<T>.scanTransform(
    initial: S,
    @BuilderInference crossinline transform: suspend FlowCollector<R>.(value: T, state: S) -> S,
): Flow<R> = flow {
  var state = initial
  collect { elem -> state = transform(this, elem, state) }
}