feat(library/tactic/simplify): add single_pass simplifier option (default is false)

doc/changes.md

@@ -5,6 +5,9 @@ master branch (aka work in progress branch)
 
 * In addition to user-defined notation parsers introduced in Lean 3.2.0, users may now also define top-level commands in Lean. For an example, see the [`coinductive` command](https://github.com/leanprover/lean/blob/814a5edaf172c3835c000e3f631bddd85bd879ab/library/init/meta/coinductive_predicates.lean#L551-L552) that has been ported to the new model.
 
+* Add new simplifier configuration option `simp_config.single_pass`. It is useful for simplification rules that may produce non-termination.
+  Example: `simp {single_pass := tt}`
+
 *Changes*
 
 * We now have two type classes for converting to string: `has_to_string` and `has_repr`.

library/init/meta/simp_tactic.lean

@@ -203,6 +203,7 @@ structure simp_config :=
 (beta : bool               := tt)
 (eta  : bool               := tt)
 (proj : bool               := tt) -- reduce projections
+(single_pass : bool        := ff)
 
 meta constant simplify_core
   (c : simp_config)

src/library/tactic/simplify.cpp

@@ -71,7 +71,8 @@ simp_config::simp_config():
     m_zeta(false),
     m_beta(false),
     m_eta(true),
-    m_proj(true) {
+    m_proj(true),
+    m_single_pass(false) {
 }
 
 simp_config::simp_config(vm_obj const & obj) {
@@ -85,6 +86,7 @@ simp_config::simp_config(vm_obj const & obj) {
     m_beta               = to_bool(cfield(obj, 7));
     m_eta                = to_bool(cfield(obj, 8));
     m_proj               = to_bool(cfield(obj, 9));
+    m_single_pass        = to_bool(cfield(obj, 10));
 }
 
 /* -----------------------------------
@@ -696,8 +698,10 @@ simp_result simplify_core_fn::visit(expr const & e, optional<expr> const & paren
             } else if (r2->first.get_new() == curr_result.get_new()) {
                 break;
             } else {
-                /* continue simplifying */
                 curr_result = join(new_result, r2->first);
+                if (m_cfg.m_single_pass)
+                    break;
+                /* continue simplifying */
             }
         } else {
             curr_result = new_result;

src/library/tactic/simplify.h

@@ -25,6 +25,7 @@ structure simp_config :=
 (beta : bool)
 (eta  : bool)
 (proj : bool)
+(single_pass : bool)
 */
 struct simp_config {
     unsigned                  m_max_steps;
@@ -37,6 +38,7 @@ struct simp_config {
     bool                      m_beta;
     bool                      m_eta;
     bool                      m_proj;
+    bool                      m_single_pass;
     simp_config();
     simp_config(vm_obj const & o);
 };

tests/lean/run/simp_single_pass.lean

@@ -0,0 +1,14 @@
+open nat well_founded
+
+def gcd : ℕ → ℕ → ℕ | y := λ x,
+if h : y = 0 then
+    x
+else
+    have x % y < y, by { apply mod_lt, cases y, contradiction, apply succ_pos },
+    gcd (x % y) y
+
+lemma gcd_zero_right (x : nat) : gcd 0 x = x :=
+begin
+  simp [gcd] {single_pass := tt},
+  simp
+end