Add cartProd to util

src/Util.md

@@ -7,6 +7,7 @@ module Util
 
 import Data.SortedSet
 import Data.String
+import Data.List.Lazy
 
 %default total
 ```
@@ -98,3 +99,23 @@ We first check to see if the needle is a prefix of the top level haystack, befor
           then True
           else isSubstr' str | x
 ```
+
+## Lazy List
+
+### Cartesian product
+
+Lazily take the cartesian product of two foldables
+
+```idris
+export
+cartProd : Foldable a => Foldable b => a e -> b f -> LazyList (e, f)
+cartProd x y = 
+  let y = foldToLazy y
+  in foldr (\e, acc => combine e y acc) [] x
+  where
+    foldToLazy : Foldable a' => a' e' -> LazyList e'
+    foldToLazy x = foldr (\e, acc => e :: acc) [] x
+    combine : e -> LazyList f -> LazyList (e, f) -> LazyList (e, f)
+    combine x [] rest = rest
+    combine x (y :: ys) rest = (x, y) :: combine x ys rest
+```