feat(kernel): add infer implicit, and use it to infer implicit arguments of inductive datatype eliminators, and tag whether parameters should be implicit or not in introduction rules in the module inductive_cmd

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/standard/logic.lean

@@ -4,7 +4,7 @@ inductive false : Bool :=
 -- No constructors
 
 theorem false_elim (c : Bool) (H : false)
-:= @false_rec c H
+:= false_rec c H
 
 inductive true : Bool :=
 | trivial : true
@@ -54,13 +54,10 @@ theorem or_elim (a b c : Bool) (H1 : a ∨ b) (H2 : a → c) (H3 : b → c) : c
 := or_rec H2 H3 H1
 
 inductive eq {A : Type} (a : A) : A → Bool :=
-| eq_intro : eq A a a  -- TODO: use elaborator in inductive_cmd module, we should not need to type A here
+| refl : eq A a a  -- TODO: use elaborator in inductive_cmd module, we should not need to type A here
 
 infix `=` 50 := eq
 
-theorem refl {A : Type} (a : A) : a = a
-:= @eq_intro A a
-
 theorem subst {A : Type} {a b : A} {P : A → Bool} (H1 : a = b) (H2 : P a) : P b
 := eq_rec H2 H1
 

src/frontends/lean/inductive_cmd.cpp

@@ -10,6 +10,7 @@ Author: Leonardo de Moura
 #include "kernel/type_checker.h"
 #include "kernel/instantiate.h"
 #include "kernel/inductive/inductive.h"
+#include "kernel/free_vars.h"
 #include "library/scoped_ext.h"
 #include "library/locals.h"
 #include "library/placeholder.h"
@@ -24,6 +25,8 @@ static name g_assign(":=");
 static name g_with("with");
 static name g_colon(":");
 static name g_bar("|");
+static name g_lcurly("{");
+static name g_rcurly("}");
 
 using inductive::intro_rule;
 using inductive::inductive_decl;
@@ -33,12 +36,6 @@ using inductive::inductive_decl_intros;
 using inductive::intro_rule_name;
 using inductive::intro_rule_type;
 
-// Mark all parameters as implicit
-static void make_implicit(buffer<parameter> & ps) {
-    for (parameter & p : ps)
-        p.m_bi = mk_implicit_binder_info();
-}
-
 // Make sure that every inductive datatype (in decls) occurring in \c type has
 // the universe levels \c lvl_params and section parameters \c section_params
 static expr fix_inductive_occs(expr const & type, buffer<inductive_decl> const & decls,
@@ -181,6 +178,8 @@ environment inductive_cmd(parser & p) {
     bool                   first = true;
     buffer<name>           ls_buffer;
     name_map<name>         id_to_short_id;
+    // store intro rule name that are markes for relaxed implicit argument inference.
+    name_set               relaxed_implicit_inference;
     unsigned               num_params = 0;
     bool                   explicit_levels = false;
     buffer<inductive_decl> decls;
@@ -250,7 +249,6 @@ environment inductive_cmd(parser & p) {
                                    params_pos);
             }
         }
-        make_implicit(ps); // parameters are implicit for introduction rules
         // parse introduction rules
         p.check_token_next(g_assign, "invalid inductive declaration, ':=' expected");
         buffer<intro_rule> intros;
@@ -260,9 +258,17 @@ environment inductive_cmd(parser & p) {
             check_atomic(intro_id);
             name full_intro_id = ns + intro_id;
             id_to_short_id.insert(full_intro_id, intro_id);
+            bool strict = true;
+            if (p.curr_is_token(g_lcurly)) {
+                p.next();
+                p.check_token_next(g_rcurly, "invalid introduction rule, '}' expected");
+                strict = false;
+                relaxed_implicit_inference.insert(full_intro_id);
+            }
             p.check_token_next(g_colon, "invalid introduction rule, ':' expected");
             expr intro_type = p.parse_scoped_expr(ps, lenv);
             intro_type = p.pi_abstract(ps, intro_type);
+            intro_type = infer_implicit(intro_type, ps.size(), strict);
             intros.push_back(intro_rule(full_intro_id, intro_type));
         }
         decls.push_back(inductive_decl(full_id, type, to_list(intros.begin(), intros.end())));
@@ -294,7 +300,6 @@ environment inductive_cmd(parser & p) {
                            p.pi_abstract(section_params, inductive_decl_type(d)),
                            inductive_decl_intros(d));
     }
-    make_implicit(section_params);
     // Add section_params to introduction rules type, and also "fix"
     // occurrences of inductive types.
     for (inductive_decl & d : decls) {
@@ -303,6 +308,8 @@ environment inductive_cmd(parser & p) {
             expr type = intro_rule_type(ir);
             type = fix_inductive_occs(type, decls, ls_buffer, section_params);
             type = p.pi_abstract(section_params, type);
+            bool strict = relaxed_implicit_inference.contains(intro_rule_name(ir));
+            type = infer_implicit(type, section_params.size(), strict);
             new_irs.push_back(intro_rule(intro_rule_name(ir), type));
         }
         d = inductive_decl(inductive_decl_name(d),

src/kernel/expr.cpp

@@ -489,6 +489,13 @@ expr update_binding(expr const & e, expr const & new_domain, expr const & new_bo
         return e;
 }
 
+expr update_binding(expr const & e, expr const & new_domain, expr const & new_body, binder_info const & bi) {
+    if (!is_eqp(binding_domain(e), new_domain) || !is_eqp(binding_body(e), new_body) || bi != binding_info(e))
+        return copy_tag(e, mk_binding(e.kind(), binding_name(e), new_domain, new_body, bi));
+    else
+        return e;
+}
+
 expr update_mlocal(expr const & e, expr const & new_type) {
     if (is_eqp(mlocal_type(e), new_type))
         return e;
@@ -583,4 +590,35 @@ static macro_definition g_let_macro_definition(new let_macro_definition_cell());
 expr mk_let_macro(expr const & e) { return mk_macro(g_let_macro_definition, 1, &e); }
 bool is_let_macro(expr const & e) { return is_macro(e) && macro_def(e) == g_let_macro_definition; }
 expr let_macro_arg(expr const & e) { lean_assert(is_let_macro(e)); return macro_arg(e, 0); }
+
+static bool has_free_var_in_domain(expr const & b, unsigned vidx) {
+    if (is_pi(b)) {
+        return has_free_var(binding_domain(b), vidx) || has_free_var_in_domain(binding_body(b), vidx+1);
+    } else {
+        return false;
+    }
+}
+
+expr infer_implicit(expr const & t, unsigned num_params, bool strict) {
+    if (num_params == 0) {
+        return t;
+    } else if (is_pi(t)) {
+        expr new_body = infer_implicit(binding_body(t), num_params-1, strict);
+        if (binding_info(t).is_implicit() || binding_info(t).is_strict_implicit()) {
+            // argument is already marked as implicit
+            return update_binding(t, binding_domain(t), new_body);
+        } else if ((strict  && has_free_var_in_domain(new_body, 0)) ||
+                   (!strict && has_free_var(new_body, 0))) {
+            return update_binding(t, binding_domain(t), new_body, mk_implicit_binder_info());
+        } else {
+            return update_binding(t, binding_domain(t), new_body);
+        }
+    } else {
+        return t;
+    }
+}
+
+expr infer_implicit(expr const & t, bool strict) {
+    return infer_implicit(t, std::numeric_limits<unsigned>::max(), strict);
+}
 }

src/kernel/expr.h

@@ -641,6 +641,7 @@ expr update_app(expr const & e, expr const & new_fn, expr const & new_arg);
 expr update_rev_app(expr const & e, unsigned num, expr const * new_args);
 template<typename C> expr update_rev_app(expr const & e, C const & c) { return update_rev_app(e, c.size(), c.data()); }
 expr update_binding(expr const & e, expr const & new_domain, expr const & new_body);
+expr update_binding(expr const & e, expr const & new_domain, expr const & new_body, binder_info const & bi);
 expr update_mlocal(expr const & e, expr const & new_type);
 expr update_sort(expr const & e, level const & new_level);
 expr update_constant(expr const & e, levels const & new_levels);
@@ -655,5 +656,17 @@ expr let_macro_arg(expr const & e);
 std::string const & get_let_macro_opcode();
 // =======================================
 
+// =======================================
+// Implicit argument inference
+/**
+   \brief Given \c t of the form <tt>Pi (x_1 : A_1) ... (x_k : A_k), B</tt>,
+   mark the first \c num_params as implicit if they are not already marked, and
+   they occur in the remaining arguments. If \c strict is false, then we
+   also mark it implicit if it occurs in \c B.
+*/
+expr infer_implicit(expr const & t, unsigned num_params, bool strict);
+expr infer_implicit(expr const & t, bool strict);
+// =======================================
+
 std::ostream & operator<<(std::ostream & out, expr const & e);
 }

src/kernel/inductive/inductive.cpp

@@ -622,15 +622,10 @@ struct add_inductive_fn {
         expr C_app   = mk_app(info.m_C, info.m_indices);
         if (m_dep_elim)
             C_app = mk_app(C_app, info.m_major_premise);
-
         expr elim_ty = Pi(info.m_major_premise, C_app);
-        unsigned i = info.m_indices.size();
-        while (i > 0) {
-            --i;
-            elim_ty = Pi(info.m_indices[i], elim_ty, mk_implicit_binder_info());
-        }
+        elim_ty   = Pi(info.m_indices, elim_ty);
         // abstract all introduction rules
-        i = get_num_its();
+        unsigned i = get_num_its();
         while (i > 0) {
             --i;
             unsigned j = m_elim_info[i].m_minor_premises.size();
@@ -643,13 +638,10 @@ struct add_inductive_fn {
         i = get_num_its();
         while (i > 0) {
             --i;
-            elim_ty = Pi(m_elim_info[i].m_C, elim_ty, mk_implicit_binder_info());
-        }
-        i = m_param_consts.size();
-        while (i > 0) {
-            --i;
-            elim_ty = Pi(m_param_consts[i], elim_ty, mk_implicit_binder_info());
+            elim_ty = Pi(m_elim_info[i].m_C, elim_ty);
         }
+        elim_ty   = Pi(m_param_consts, elim_ty);
+        elim_ty   = infer_implicit(elim_ty, true /* strict */);
         m_env = m_env.add(check(m_env, mk_var_decl(get_elim_name(d), get_elim_level_param_names(), elim_ty)));
     }
 

tests/lean/run/e14.lean

@@ -3,8 +3,8 @@ inductive nat : Type :=
 | succ : nat → nat
 
 inductive list (A : Type) : Type :=
-| nil  : list A
-| cons : A → list A → list A
+| nil {} : list A
+| cons   : A → list A → list A
 
 check nil
 check nil.{1}
@@ -14,8 +14,8 @@ check @nil nat
 check cons zero nil
 
 inductive vector (A : Type) : nat → Type :=
-| vnil  : vector A zero
-| vcons : forall {n : nat}, A → vector A n → vector A (succ n)
+| vnil {} : vector A zero
+| vcons   : forall {n : nat}, A → vector A n → vector A (succ n)
 
 check vcons zero vnil
 variable n : nat

tests/lean/run/e15.lean

@@ -3,8 +3,8 @@ inductive nat : Type :=
 | succ : nat → nat
 
 inductive list (A : Type) : Type :=
-| nil  : list A
-| cons : A → list A → list A
+| nil {} : list A
+| cons   : A → list A → list A
 
 check nil
 check nil.{1}
@@ -14,8 +14,8 @@ check @nil nat
 check cons zero nil
 
 inductive vector (A : Type) : nat → Type :=
-| vnil  : vector A zero
-| vcons : forall {n : nat}, A → vector A n → vector A (succ n)
+| vnil {} : vector A zero
+| vcons   : forall {n : nat}, A → vector A n → vector A (succ n)
 
 check vcons zero vnil
 variable n : nat
@@ -25,4 +25,3 @@ check vector_rec
 
 definition vector_to_list {A : Type} {n : nat} (v : vector A n) : list A
 := vector_rec nil (fun (n : nat) (a : A) (v : vector A n) (l : list A), cons a l) v
-

tests/lean/run/e16.lean

@@ -3,8 +3,8 @@ inductive nat : Type :=
 | succ : nat → nat
 
 inductive list (A : Type) : Type :=
-| nil  : list A
-| cons : A → list A → list A
+| nil {} : list A
+| cons   : A → list A → list A
 
 check nil
 check nil.{1}
@@ -14,8 +14,8 @@ check @nil nat
 check cons zero nil
 
 inductive vector (A : Type) : nat → Type :=
-| vnil  : vector A zero
-| vcons : forall {n : nat}, A → vector A n → vector A (succ n)
+| vnil {} : vector A zero
+| vcons   : forall {n : nat}, A → vector A n → vector A (succ n)
 
 check vcons zero vnil
 variable n : nat
@@ -25,4 +25,3 @@ check vector_rec
 
 definition vector_to_list {A : Type} {n : nat} (v : vector A n) : list A
 := vector_rec nil (fun n a v l, cons a l) v
-

tests/lean/run/e17.lean

@@ -3,8 +3,8 @@ inductive nat : Type :=
 | succ : nat → nat
 
 inductive list (A : Type) : Type :=
-| nil  : list A
-| cons : A → list A → list A
+| nil {} : list A
+| cons   : A → list A → list A
 
 inductive int : Type :=
 | of_nat : nat → int