Add foldl example

.gitignore

@@ -2,6 +2,7 @@
 /lua_modules
 /.luarocks
 /.out
+/.ccls*
 /tree
 **.o
 **.so

lua/functional/foldl.lua

@@ -0,0 +1,18 @@
+-- This is equivalent to the Haskell foldl:
+-- foldl :: (b -> a -> b) -> b -> [a] -> b
+-- foldl f z []     = z
+-- foldl f z (x:xs) = foldl f (f z x) xs
+
+
+
+local foldl
+function foldl(f, z, xs, i)
+  if (i or 0) >= #xs then return z end
+  i = (i or 0) + 1
+  return foldl(f, f(z, xs[i]), xs, i)
+end
+
+local sum
+function sum(a, b) return a + b end
+
+print( assert( foldl(sum,0,{1,2,3,5,8,13,21,34,55,89}) == 231 ) )