chore(tests/playground): add zipper

tests/playground/zipper.lean

@@ -0,0 +1,61 @@
+structure ListZipper (α : Type) :=
+(xs : List α) (bs : List α)
+
+-- set_option trace.compiler.ir.rc true
+
+variables {α : Type}
+
+namespace ListZipper
+
+def goForward : ListZipper α → ListZipper α
+| ⟨[], bs⟩    := ⟨[], bs⟩
+| ⟨x::xs, bs⟩ := ⟨xs, x::bs⟩
+
+def goBackward : ListZipper α → ListZipper α
+| ⟨xs, []⟩    := ⟨xs, []⟩
+| ⟨xs, b::bs⟩ := ⟨b::xs, bs⟩
+
+def insert : ListZipper α → α → ListZipper α
+| ⟨xs, bs⟩ x := ⟨xs, x::bs⟩
+
+def erase : ListZipper α → ListZipper α
+| ⟨[], bs⟩ := ⟨[], bs⟩
+| ⟨x::xs, bs⟩ := ⟨xs, bs⟩
+
+def curr [Inhabited α] : ListZipper α → α
+| ⟨[], bs⟩    := default _
+| ⟨x::xs, bs⟩ := x
+
+def currOpt : ListZipper α → Option α
+| ⟨[], bs⟩    := none
+| ⟨x::xs, bs⟩ := some x
+
+def toList : ListZipper α → List α
+| ⟨xs, bs⟩ := bs.reverse ++ xs
+
+def atEnd (z : ListZipper α) : Bool :=
+z.xs.isEmpty
+
+end ListZipper
+
+def List.toListZipper (xs : List α) : ListZipper α :=
+⟨xs, []⟩
+
+partial def testAux : ListZipper Nat → ListZipper Nat
+| z :=
+  if z.atEnd then
+    z
+  else if z.curr % 2 == 0 then
+    testAux (z.goForward.insert 0)
+  else if z.curr > 20 then
+    testAux z.erase
+  else
+    testAux z.goForward
+
+def test (xs : List Nat) : List Nat :=
+(testAux xs.toListZipper).toList
+
+def main (xs : List String) : IO Unit :=
+do
+IO.println $ test [10, 11, 13, 20, 22, 40, 41, 11],
+IO.println $ test (xs.map String.toNat)