feat(library/init/data/list/qsort): add temp qsort as meta definition

library/init/data/list/default.lean

@@ -4,4 +4,4 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Leonardo de Moura
 -/
 prelude
-import init.data.list.basic init.data.list.instances init.data.list.lemmas
+import init.data.list.basic init.data.list.instances init.data.list.lemmas init.data.list.qsort

library/init/data/list/qsort.lean

@@ -0,0 +1,23 @@
+/-
+Copyright (c) 2017 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Author: Leonardo de Moura
+-/
+prelude
+import init.data.list.basic
+
+namespace list
+
+/- TODO(Leo): remove meta as soon as equation compiler has support for well founded recursion.
+
+   This is based on the minimalist Haskell "quicksort".
+
+   Remark: this is *not* really quicksort since it doesn't partition the elements in-place -/
+meta def qsort {α} (lt : α → α → bool) : list α → list α
+| []     := []
+| (h::t) :=
+  let small := filter (λ x, lt h x = ff) t,
+      large := filter (λ x, lt h x = tt) t
+  in qsort small ++ h :: qsort large
+
+end list