test(tests/playground/mapVShmap): add example where hmap is almost 3x faster than map

tests/playground/mapVShmap.lean

@@ -0,0 +1,34 @@
+def mkBigPairs : Nat → Array (Nat × Nat) → Array (Nat × Nat)
+| 0     ps := ps
+| (n+1) ps := mkBigPairs n (ps.push (n, n))
+
+set_option pp.implicit true
+set_option pp.binder_types false
+-- set_option trace.compiler.specialize true
+-- set_option trace.compiler.boxed true
+
+def f1 (ps : Array (Nat × Nat)) : Array (Nat × Nat) :=
+ps.hmap (λ p, match p with (n, m) := (n+1, m))
+
+def f2 (ps : Array (Nat × Nat)) : Array (Nat × Nat) :=
+ps.map (λ p, match p with (n, m) := (n+1, m))
+
+def prof {α : Type} (msg : String) (p : IO α) : IO α :=
+let msg₁ := "Time for '" ++ msg ++ "':" in
+timeit msg₁ p
+
+def test1 (n : Nat) (m : Array (Nat × Nat)) : IO Unit :=
+let m := n.repeat f1 m in
+let s := m.foldl (λ p n, n + p.1) 0 in
+IO.println s
+
+def test2 (n : Nat) (m : Array (Nat × Nat)) : IO Unit :=
+let m := n.repeat f2 m in
+let s := m.foldl (λ p n, n + p.1) 0 in
+IO.println s
+
+def main (xs : List String) : IO Unit :=
+let n := xs.head.toNat in
+let m := mkBigPairs n ∅ in
+prof "hmap" (test1 n m) *>
+prof "map"  (test2 n m)