diff --git a/library/standard.lean b/library/standard.lean
index 867b46c60a..1f9f0e2a3c 100644
--- a/library/standard.lean
+++ b/library/standard.lean
@@ -3,4 +3,4 @@ Copyright (c) 2014 Microsoft Corporation. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Leonardo de Moura, Jeremy Avigad
 -/
-import system.IO
+import system.io
diff --git a/library/system/IO.lean b/library/system/IO.lean
deleted file mode 100644
index 2d9133df9b..0000000000
--- a/library/system/IO.lean
+++ /dev/null
@@ -1,25 +0,0 @@
-/-
-Copyright (c) 2016 Microsoft Corporation. All rights reserved.
-Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Luke Nelson, Jared Roesch and Leonardo de Moura
--/
-constant {u} IO : Type u → Type u
-constant functorIO : functor IO
-constant monadIO : monad IO
-
-attribute [instance] functorIO monadIO
-
-constant put_str : string → IO unit
-constant put_nat : nat → IO unit
-constant get_line : IO string
-
-meta constant format.print_using : format → options → IO unit
-
-meta definition format.print (fmt : format) : IO unit :=
-format.print_using fmt options.mk
-
-meta definition pp_using {A : Type} [has_to_format A] (a : A) (o : options) : IO unit :=
-format.print_using (to_fmt a) o
-
-meta definition pp {A : Type} [has_to_format A] (a : A) : IO unit :=
-format.print (to_fmt a)
diff --git a/library/system/io.lean b/library/system/io.lean
new file mode 100644
index 0000000000..188e435071
--- /dev/null
+++ b/library/system/io.lean
@@ -0,0 +1,25 @@
+/-
+Copyright (c) 2016 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Luke Nelson, Jared Roesch and Leonardo de Moura
+-/
+constant {u} io : Type u → Type u
+constant io.functor : functor io
+constant io.monad   : monad io
+
+attribute [instance] io.functor io.monad
+
+constant put_str  : string → io unit
+constant put_nat  : nat → io unit
+constant get_line : io string
+
+meta constant format.print_using : format → options → io unit
+
+meta definition format.print (fmt : format) : io unit :=
+format.print_using fmt options.mk
+
+meta definition pp_using {A : Type} [has_to_format A] (a : A) (o : options) : io unit :=
+format.print_using (to_fmt a) o
+
+meta definition pp {A : Type} [has_to_format A] (a : A) : io unit :=
+format.print (to_fmt a)
diff --git a/src/frontends/lean/builtin_cmds.cpp b/src/frontends/lean/builtin_cmds.cpp
index f50905ebc7..902d563aec 100644
--- a/src/frontends/lean/builtin_cmds.cpp
+++ b/src/frontends/lean/builtin_cmds.cpp
@@ -439,9 +439,9 @@ static environment vm_eval_cmd(parser & p) {
     type_context tc(p.env(), transparency_mode::All);
     expr type0 = tc.infer(e);
     expr type  = tc.whnf(type0);
-    bool is_IO = is_constant(get_app_fn(type), get_IO_name());
+    bool is_io = is_constant(get_app_fn(type), get_io_name());
     bool is_string = false;
-    if (!is_IO) {
+    if (!is_io) {
         /* Check if resultant type has an instance of has_to_string */
         level lvl  = sort_level(tc.whnf(tc.infer(type)));
         expr has_to_string_type = mk_app(mk_constant(get_has_to_string_name(), {lvl}), type0);
@@ -457,7 +457,7 @@ static environment vm_eval_cmd(parser & p) {
     environment new_env = compile_expr(p.env(), main, ls, type, e, pos);
     vm_state s(new_env, p.get_options());
     optional<vm_obj> initial_state;
-    if (is_IO) initial_state = mk_vm_simple(0);
+    if (is_io) initial_state = mk_vm_simple(0);
     auto out = p.mk_message(pos, INFORMATION);
     out.set_caption("vm_eval result");
     vm_state::profiler prof(s, p.get_options());
@@ -468,7 +468,7 @@ static environment vm_eval_cmd(parser & p) {
     } else {
         vm_eval_core(s, main, initial_state);
     }
-    if (is_IO) {
+    if (is_io) {
         // do not print anything
     } else if (is_string) {
         vm_obj r = s.get(0);
diff --git a/src/library/compiler/erase_irrelevant.cpp b/src/library/compiler/erase_irrelevant.cpp
index 4c2b3a0561..dfcaa771cc 100644
--- a/src/library/compiler/erase_irrelevant.cpp
+++ b/src/library/compiler/erase_irrelevant.cpp
@@ -302,7 +302,7 @@ class erase_irrelevant_fn : public compiler_step_visitor {
     }
 
     static bool is_builtin_state_monad(expr const & e) {
-        return is_constant(e, get_monadIO_name());
+        return is_constant(e, get_io_monad_name());
     }
 
     expr visit_monad_bind(expr const & e, buffer<expr> const & args) {
diff --git a/src/library/constants.cpp b/src/library/constants.cpp
index c62ba0cf84..c6b125a041 100644
--- a/src/library/constants.cpp
+++ b/src/library/constants.cpp
@@ -72,7 +72,6 @@ name const * g_eq_rec_heq = nullptr;
 name const * g_exists_elim = nullptr;
 name const * g_format = nullptr;
 name const * g_functor = nullptr;
-name const * g_functorIO = nullptr;
 name const * g_false = nullptr;
 name const * g_false_of_true_iff_false = nullptr;
 name const * g_false_rec = nullptr;
@@ -142,7 +141,9 @@ name const * g_int_has_lt = nullptr;
 name const * g_int_has_neg = nullptr;
 name const * g_int_has_mod = nullptr;
 name const * g_int_decidable_linear_ordered_comm_group = nullptr;
-name const * g_IO = nullptr;
+name const * g_io = nullptr;
+name const * g_io_functor = nullptr;
+name const * g_io_monad = nullptr;
 name const * g_is_associative = nullptr;
 name const * g_is_associative_op_assoc = nullptr;
 name const * g_is_int = nullptr;
@@ -174,7 +175,6 @@ name const * g_monad_map = nullptr;
 name const * g_monad_bind = nullptr;
 name const * g_monad_ret = nullptr;
 name const * g_monad_and_then = nullptr;
-name const * g_monadIO = nullptr;
 name const * g_monoid = nullptr;
 name const * g_mul = nullptr;
 name const * g_mul_one = nullptr;
@@ -464,7 +464,6 @@ void initialize_constants() {
     g_exists_elim = new name{"exists", "elim"};
     g_format = new name{"format"};
     g_functor = new name{"functor"};
-    g_functorIO = new name{"functorIO"};
     g_false = new name{"false"};
     g_false_of_true_iff_false = new name{"false_of_true_iff_false"};
     g_false_rec = new name{"false", "rec"};
@@ -534,7 +533,9 @@ void initialize_constants() {
     g_int_has_neg = new name{"int", "has_neg"};
     g_int_has_mod = new name{"int", "has_mod"};
     g_int_decidable_linear_ordered_comm_group = new name{"int_decidable_linear_ordered_comm_group"};
-    g_IO = new name{"IO"};
+    g_io = new name{"io"};
+    g_io_functor = new name{"io", "functor"};
+    g_io_monad = new name{"io", "monad"};
     g_is_associative = new name{"is_associative"};
     g_is_associative_op_assoc = new name{"is_associative", "op_assoc"};
     g_is_int = new name{"is_int"};
@@ -566,7 +567,6 @@ void initialize_constants() {
     g_monad_bind = new name{"monad", "bind"};
     g_monad_ret = new name{"monad", "ret"};
     g_monad_and_then = new name{"monad", "and_then"};
-    g_monadIO = new name{"monadIO"};
     g_monoid = new name{"monoid"};
     g_mul = new name{"mul"};
     g_mul_one = new name{"mul_one"};
@@ -857,7 +857,6 @@ void finalize_constants() {
     delete g_exists_elim;
     delete g_format;
     delete g_functor;
-    delete g_functorIO;
     delete g_false;
     delete g_false_of_true_iff_false;
     delete g_false_rec;
@@ -927,7 +926,9 @@ void finalize_constants() {
     delete g_int_has_neg;
     delete g_int_has_mod;
     delete g_int_decidable_linear_ordered_comm_group;
-    delete g_IO;
+    delete g_io;
+    delete g_io_functor;
+    delete g_io_monad;
     delete g_is_associative;
     delete g_is_associative_op_assoc;
     delete g_is_int;
@@ -959,7 +960,6 @@ void finalize_constants() {
     delete g_monad_bind;
     delete g_monad_ret;
     delete g_monad_and_then;
-    delete g_monadIO;
     delete g_monoid;
     delete g_mul;
     delete g_mul_one;
@@ -1249,7 +1249,6 @@ name const & get_eq_rec_heq_name() { return *g_eq_rec_heq; }
 name const & get_exists_elim_name() { return *g_exists_elim; }
 name const & get_format_name() { return *g_format; }
 name const & get_functor_name() { return *g_functor; }
-name const & get_functorIO_name() { return *g_functorIO; }
 name const & get_false_name() { return *g_false; }
 name const & get_false_of_true_iff_false_name() { return *g_false_of_true_iff_false; }
 name const & get_false_rec_name() { return *g_false_rec; }
@@ -1319,7 +1318,9 @@ name const & get_int_has_lt_name() { return *g_int_has_lt; }
 name const & get_int_has_neg_name() { return *g_int_has_neg; }
 name const & get_int_has_mod_name() { return *g_int_has_mod; }
 name const & get_int_decidable_linear_ordered_comm_group_name() { return *g_int_decidable_linear_ordered_comm_group; }
-name const & get_IO_name() { return *g_IO; }
+name const & get_io_name() { return *g_io; }
+name const & get_io_functor_name() { return *g_io_functor; }
+name const & get_io_monad_name() { return *g_io_monad; }
 name const & get_is_associative_name() { return *g_is_associative; }
 name const & get_is_associative_op_assoc_name() { return *g_is_associative_op_assoc; }
 name const & get_is_int_name() { return *g_is_int; }
@@ -1351,7 +1352,6 @@ name const & get_monad_map_name() { return *g_monad_map; }
 name const & get_monad_bind_name() { return *g_monad_bind; }
 name const & get_monad_ret_name() { return *g_monad_ret; }
 name const & get_monad_and_then_name() { return *g_monad_and_then; }
-name const & get_monadIO_name() { return *g_monadIO; }
 name const & get_monoid_name() { return *g_monoid; }
 name const & get_mul_name() { return *g_mul; }
 name const & get_mul_one_name() { return *g_mul_one; }
diff --git a/src/library/constants.h b/src/library/constants.h
index e2ff31f46e..9f53dbb4d5 100644
--- a/src/library/constants.h
+++ b/src/library/constants.h
@@ -74,7 +74,6 @@ name const & get_eq_rec_heq_name();
 name const & get_exists_elim_name();
 name const & get_format_name();
 name const & get_functor_name();
-name const & get_functorIO_name();
 name const & get_false_name();
 name const & get_false_of_true_iff_false_name();
 name const & get_false_rec_name();
@@ -144,7 +143,9 @@ name const & get_int_has_lt_name();
 name const & get_int_has_neg_name();
 name const & get_int_has_mod_name();
 name const & get_int_decidable_linear_ordered_comm_group_name();
-name const & get_IO_name();
+name const & get_io_name();
+name const & get_io_functor_name();
+name const & get_io_monad_name();
 name const & get_is_associative_name();
 name const & get_is_associative_op_assoc_name();
 name const & get_is_int_name();
@@ -176,7 +177,6 @@ name const & get_monad_map_name();
 name const & get_monad_bind_name();
 name const & get_monad_ret_name();
 name const & get_monad_and_then_name();
-name const & get_monadIO_name();
 name const & get_monoid_name();
 name const & get_mul_name();
 name const & get_mul_one_name();
diff --git a/src/library/constants.txt b/src/library/constants.txt
index b6f9e935ed..89a67b7b60 100644
--- a/src/library/constants.txt
+++ b/src/library/constants.txt
@@ -67,7 +67,6 @@ eq_rec_heq
 exists.elim
 format
 functor
-functorIO
 false
 false_of_true_iff_false
 false.rec
@@ -137,7 +136,9 @@ int.has_lt
 int.has_neg
 int.has_mod
 int_decidable_linear_ordered_comm_group
-IO
+io
+io.functor
+io.monad
 is_associative
 is_associative.op_assoc
 is_int
@@ -169,7 +170,6 @@ monad.map
 monad.bind
 monad.ret
 monad.and_then
-monadIO
 monoid
 mul
 mul_one
diff --git a/src/library/library_system.cpp b/src/library/library_system.cpp
index d9199eb8f8..59853a1b10 100644
--- a/src/library/library_system.cpp
+++ b/src/library/library_system.cpp
@@ -10,9 +10,9 @@ Author: Leonardo de Moura
 namespace lean {
 bool is_system_builtin(name const & n) {
     return
-        n == get_IO_name() ||
-        n == get_functorIO_name() ||
-        n == get_monadIO_name() ||
+        n == get_io_name() ||
+        n == get_io_functor_name() ||
+        n == get_io_monad_name() ||
         n == get_put_str_name() ||
         n == get_put_nat_name() ||
         n == get_get_line_name();
diff --git a/tests/lean/char_lits.lean b/tests/lean/char_lits.lean
index 51b68503d6..eb7056c438 100644
--- a/tests/lean/char_lits.lean
+++ b/tests/lean/char_lits.lean
@@ -1,4 +1,4 @@
-import system.IO
+import system.io
 
 check 'a'
 
diff --git a/tests/lean/run/IO1.lean b/tests/lean/run/IO1.lean
index d8a0b9220d..2e1ca439c6 100644
--- a/tests/lean/run/IO1.lean
+++ b/tests/lean/run/IO1.lean
@@ -1,9 +1,9 @@
-import system.IO
+import system.io
 open list
 
 -- set_option pp.all true
 
-definition main : IO unit :=
+definition main : io unit :=
   do l₁ ← get_line,
      l₂ ← get_line,
      put_str (l₂ ++ l₁)
@@ -15,7 +15,7 @@ vm_eval put_str "hello\n"
 
 print "************************"
 
-definition aux (n : nat) : IO unit :=
+definition aux (n : nat) : io unit :=
   do put_str "========\nvalue: ",
      put_nat n,
      put_str "\n========\n"
@@ -24,7 +24,7 @@ vm_eval aux 20
 
 print "************************"
 
-definition repeat : nat → (nat → IO unit) → IO unit
+definition repeat : nat → (nat → io unit) → io unit
 | 0     a := return ()
 | (n+1) a := do a n, repeat n a
 
@@ -32,7 +32,7 @@ vm_eval repeat 10 aux
 
 print "************************"
 
-definition execute : list (IO unit) → IO unit
+definition execute : list (io unit) → io unit
 | []      := return ()
 | (x::xs) := do x, execute xs
 
diff --git a/tests/lean/run/IO2.lean b/tests/lean/run/IO2.lean
index cc0dbc7331..065ad0cdac 100644
--- a/tests/lean/run/IO2.lean
+++ b/tests/lean/run/IO2.lean
@@ -1,4 +1,4 @@
-import system.IO
+import system.io
 open list
 
 /- B and unit must be in the same universe
@@ -6,7 +6,7 @@ open list
    since unit is at Type₁
 -/
 
-definition foreach {A : Type} {B : Type} : list A → (A → IO B) → IO poly_unit
+definition foreach {A : Type} {B : Type} : list A → (A → io B) → io poly_unit
 | []      f := return poly_unit.star
 | (x::xs) f := do f x, foreach xs f
 
diff --git a/tests/lean/run/IO3.lean b/tests/lean/run/IO3.lean
index 9d3e7dd3ef..9300ee4910 100644
--- a/tests/lean/run/IO3.lean
+++ b/tests/lean/run/IO3.lean
@@ -1,6 +1,6 @@
-import system.IO
+import system.io
 
-definition main : IO unit :=
+definition main : io unit :=
 do { l ← get_line,
      if l = "hello" then
        put_str "you have typed hello\n"
diff --git a/tests/lean/run/IO4.lean b/tests/lean/run/IO4.lean
index fb9a156ad4..e64a5612e8 100644
--- a/tests/lean/run/IO4.lean
+++ b/tests/lean/run/IO4.lean
@@ -1,8 +1,8 @@
-import system.IO
+import system.io
 
 set_option trace.compiler.code_gen true
 
-definition main : IO unit :=
+definition main : io unit :=
 do { n ← return (10:nat),
      if n = (11:nat) then
        put_nat 1
diff --git a/tests/lean/run/format.lean b/tests/lean/run/format.lean
index 71fc027500..d6f61cdbfd 100644
--- a/tests/lean/run/format.lean
+++ b/tests/lean/run/format.lean
@@ -1,4 +1,4 @@
-import system.IO
+import system.io
 open list
 
 vm_eval pp ([["aaa", "bbb", "ccc", "dddd", "eeeeee", "ffffff"], ["aaa", "bbb", "ccc", "dddd", "eeeeee", "ffffff"],
diff --git a/tests/lean/run/meta2.lean b/tests/lean/run/meta2.lean
index a6327b1959..c796c24db6 100644
--- a/tests/lean/run/meta2.lean
+++ b/tests/lean/run/meta2.lean
@@ -1,9 +1,9 @@
-import system.IO
+import system.io
 
 meta definition foo : nat → nat
 | a := nat.cases_on a 1 (λ n, foo n + 2)
 
 vm_eval (foo 10)
 
-meta definition loop : nat → IO unit
+meta definition loop : nat → io unit
 | a := do put_str ">> ", put_nat a, put_str "\n", loop (a+1)
diff --git a/tests/lean/run/meta_level1.lean b/tests/lean/run/meta_level1.lean
index 2c940ae31d..bf8475dd0a 100644
--- a/tests/lean/run/meta_level1.lean
+++ b/tests/lean/run/meta_level1.lean
@@ -1,4 +1,4 @@
-import system.IO
+import system.io
 
 vm_eval do pp (level.max (level.succ level.zero) (level.param `foo)), put_str "\n"
 
diff --git a/tests/lean/run/rb_map1.lean b/tests/lean/run/rb_map1.lean
index 4308d362a4..c83063fdd8 100644
--- a/tests/lean/run/rb_map1.lean
+++ b/tests/lean/run/rb_map1.lean
@@ -1,4 +1,4 @@
-import system.IO
+import system.io
 
 section
 open nat_map
diff --git a/tests/lean/run/whenIO.lean b/tests/lean/run/whenIO.lean
index 220f43ea00..9e36074a4e 100644
--- a/tests/lean/run/whenIO.lean
+++ b/tests/lean/run/whenIO.lean
@@ -1,6 +1,6 @@
-import system.IO
+import system.io
 
-definition when (b : bool) (a : IO unit) : IO unit :=
+definition when (b : bool) (a : io unit) : io unit :=
 if b = tt then a else return ()
 
 vm_eval when tt (put_str "hello\n")