feat: add first 5 99 problems in nix

99.nix

@@ -0,0 +1,72 @@
+# Based on the Ninety-Nine Lisp Problems:
+# https://www.ic.unicamp.br/~meidanis/courses/mc336/problemas-lisp/L-99_Ninety-Nine_Lisp_Problems.html
+# and following the format on OCaml's site: https://ocaml.org/exercises
+# Solved only using `builtins.head` and `builtins.tail` to start
+
+let
+  head = builtins.head;
+  tail = builtins.tail;
+
+  # Problem 1
+in let
+  last = lst:
+    if lst == [ ] then
+      null
+    else if (tail lst) == [ ] then
+      head lst
+    else
+      last (tail lst);
+in assert last [ ] == null;
+assert last [ 1 ] == 1;
+assert last [ 1 2 3 ] == 3;
+
+# Problem 2
+let
+  last-two = lst:
+    if lst == [ ] || (tail lst) == [ ] then
+      null
+    else if (tail (tail lst)) == [ ] then
+      let
+        fst = head lst;
+        snd = head (tail lst);
+      in [ fst snd ]
+    else
+      last-two (tail lst);
+in assert last-two [ ] == null;
+assert last-two [ 1 ] == null;
+assert last-two [ 1 2 ] == [ 1 2 ];
+assert last-two [ 1 2 3 ] == [ 2 3 ];
+
+# Problem 3
+let
+  at = lst: n:
+    if lst == [ ] then
+      throw "Index out-of-bounds"
+    else if n == 0 then
+      head lst
+    else
+      at (tail lst) (n - 1);
+in assert !(builtins.tryEval (at [ ] 0)).success;
+assert !(builtins.tryEval (at [ 1 2 3 ] 4)).success;
+assert at [ 1 2 3 ] 0 == 1;
+assert at [ 1 2 3 ] 1 == 2;
+assert at [ 1 2 3 ] 2 == 3;
+
+# Problem 4
+let length = lst: if lst == [ ] then 0 else 1 + length (tail lst);
+in assert length [ ] == 0;
+assert length [ 1 ] == 1;
+assert length [ 1 2 3 ] == 3;
+
+# Problem 5
+let
+  rev = lst:
+    let
+      rev' = lst: acc:
+        if lst == [ ] then acc else rev' (tail lst) ([ (head lst) ] ++ acc);
+    in rev' lst [ ];
+in assert rev [ ] == [ ];
+assert rev [ 1 ] == [ 1 ];
+assert rev [ 1 2 3 ] == [ 3 2 1 ];
+assert rev [ 1 2 1 ] == [ 1 2 1 ];
+"ok"