feat(library/util): add auxiliary functions

src/library/util.cpp

@@ -12,6 +12,31 @@ Author: Leonardo de Moura
 #include "library/locals.h"
 
 namespace lean {
+bool is_def_app(environment const & env, expr const & e) {
+    if (!is_app(e))
+        return false;
+    expr const & f = get_app_fn(e);
+    if (!is_constant(f))
+        return false;
+    auto decl = env.find(const_name(f));
+    return decl && decl->is_definition() && !decl->is_opaque();
+}
+
+optional<expr> unfold_app(environment const & env, expr const & e) {
+    if (!is_app(e))
+        return none_expr();
+    expr const & f = get_app_fn(e);
+    if (!is_constant(f))
+        return none_expr();
+    auto decl = env.find(const_name(f));
+    if (!decl || !decl->is_definition() || decl->is_opaque())
+        return none_expr();
+    expr d = instantiate_value_univ_params(*decl, const_levels(f));
+    buffer<expr> args;
+    get_app_rev_args(e, args);
+    return some_expr(apply_beta(d, args.size(), args.data()));
+}
+
 optional<level> dec_level(level const & l) {
     switch (kind(l)) {
     case level_kind::Zero: case level_kind::Param: case level_kind::Global: case level_kind::Meta:

src/library/util.h

@@ -11,6 +11,16 @@ Author: Leonardo de Moura
 namespace lean {
 typedef std::unique_ptr<type_checker> type_checker_ptr;
 
+/** \brief Return true iff \c e is of the form <tt>(f ...)</tt> where
+    \c f is a non-opaque constant definition */
+bool is_def_app(environment const & env, expr const & e);
+
+/** \brief If \c e is of the form <tt>(f a_1 ... a_n)</tt>, where \c f is
+    a non-opaque definition, then unfold \c f, and beta reduce.
+    Otherwise, return none.
+*/
+optional<expr> unfold_app(environment const & env, expr const & e);
+
 /** \brief Reduce (if possible) universe level by 1.
     \pre is_not_zero(l)
 */