New Post

content/blog/fold-not-only-reduces.md

@@ -0,0 +1,63 @@
+---
+title: "Fold Not Only Reduces"
+date: 2022-11-09T15:15:10-05:00
+draft: false
+tags: []
+math: false
+---
+
+One misconception when first learning about fold is that it takes a list of elements of a certain type (`List[T]`) and "reduces" it to a single item of type `T`.
+
+This misconception is aided by one of the most common fold examples: summing a list.
+
+Scala Example:
+
+```scala
+List(1, 2, 3, 4, 5).foldLeft(0)((c, n) => c + n)
+// Returns 15
+```
+
+Haskell Example:
+
+```haskell
+foldl (+) 0 [1,2,3,4,5]
+-- Returns 15
+```
+
+However, let us look more closely at the type signature of `foldLeft` on a list of type `X`.
+
+Haskell:
+
+```
+(B -> X -> B) -> B -> [X] -> B
+```
+
+Scala:
+
+```
+(id: B)(op: (B, X) => B): B
+```
+
+There are a few things we can note here:
+
+- The return type is not influenced by the list type `X` at all.
+- The return type must match the type of the id of the fold.
+- The operation takes two arguments, with the first type matching the id/start (`B`) and the second type matching the type within the list (`X`)
+
+To show an example of how we don't need to "reduce", let's return the elements of a list that's greater than 5.
+
+Scala Example:
+
+```scala
+List(5, 7, 1, 8, 9, 3).foldLeft(List.empty[Int])((c, n) => if n > 5 then c :+ n else c)
+// Returns List(7, 8, 9)
+```
+
+Haskell Example:
+
+```haskell
+l5 c n if n > 5 then c ++ [n] else c
+foldl l5 [] [5, 7, 1, 8, 9, 3]
+-- Returns [7,8,9]
+```
+