In functional programming, we often look at a list in terms of its head (first-element) and tail (rest-of-list). This allows us to define operations on a list recursively. For example, how do we sum a list of integers such as `[1, 2, 3, 4]`?
```scala
def sum(l : List[Int]): Int =
if l.size == 0 then
0
else if l.size == 1 then
l.head
else
l.head + sum(l.tail)
```
We later learn that the `fold` version is more compact.
```scala
l.foldLeft(0)(_ + _)
```
The big question though, is how do we write this function if we allow lists to be arbitrarily nested? One example of this is the list `[[1, 2, [3, 4]], 5, [[6, 7], 8]]`
## Deep Recursion
To accomplish this, we need to make use of *deep recursion*. At its essence, we change the previous program so that it also recurses on the head of the list as well since that may be a list.
```scala
def deep_sum(l: Int | Matchable): Int =
if l.isInstanceOf[Int] then
l.asInstanceOf[Int]
else
val ll = l.asInstanceOf[List[Int | Matchable]]
if ll.size == 0 then
0
else if ll.size == 1 then
deep_sum(ll.head)
else
deep_sum(ll.head) + deep_sum(ll.tail)
```
Lets trace through an example `[[1], 2]`
```
deep_sum([[1], 2])
deep_sum([1]) + deep_sum([2])
deep_sum(1) + deep_sum([2])
1 + deep_sum([2])
1 + deep_sum(2)
1 + 2
3
```
## Deep Recursion via Fold
Similar to shallow recursion, we can use the `foldLeft` function to help clean up the code a little:
```scala
def deep_sum(l : Int | Matchable): Int =
if l.isInstanceOf[Int] then
l.asInstanceOf[Int]
else
val ll = l.asInstanceOf[List[Int | Matchable]]
ll.foldLeft(0)((c, n) => c + deep_sum(n))
```
In the above fold, `c` contains the current partial result (of type `Int`) which we can then add the recursive result of the next element of the list.
In the prior example, the deep recursion and the reduction logic were combined within the same anonymous function. We can separate this out by making use of `map`.
```scala
def deep_sum(l: Int | Matchable): Int =
if l.isInstanceOf[Int] then
l.asInstanceOf[Int]
else
val ll = l.asInstanceOf[List[Int | Matchable]]
l.map(deep_sum).foldLeft(_ + _)
```
Intuitively, the map will apply `deep_sum` to each element of the list and returns an `Int` for each element as that's the return type of `deep_sum`. Once we have our list of integers, we can perform the fold to reduce it to a single sum.
