diff --git a/src/library/compiler/csimp.cpp b/src/library/compiler/csimp.cpp
index 48b34bbf93..92c016b3ef 100644
--- a/src/library/compiler/csimp.cpp
+++ b/src/library/compiler/csimp.cpp
@@ -1366,6 +1366,45 @@ class csimp_fn {
         return mk_app(mk_constant(get_nat_add_name()), arg, mk_lit(literal(nat(1))));
     }
 
+    /*
+      Replace `fix_core_n f a_1 ... a_m`
+      with `fix_core_m f a_1 ... a_m` whenever `n < m`.
+
+      This optimization is for writing reusable/generic code. For
+      example, we cannot write an efficient `rec_t` monad transformer
+      without it because we don't know the arity of `m A` when we write `rec_t`.
+
+      Remark: the runtime provides a small set of `fix_core_i` implementations (`i in [1, 6]`).
+      This methods does nothing if `m > 6`. */
+    expr visit_fix_core(expr const & e, unsigned n) {
+        if (m_before_erasure) return visit_app_default(e);
+        buffer<expr> args;
+        expr fn = get_app_args(e, args);
+        lean_assert(is_constant(fn) && is_fix_core(const_name(fn)));
+        unsigned arity =
+            n + /* α_1 ... α_n Type arguments */
+            1 + /* β : Type */
+            1 + /* (base : α_1 → ... → α_n → β) */
+            1 + /* (rec : (α_1 → ... → α_n → β) → α_1 → ... → α_n → β) */
+            n; /* α_1 → ... → α_n */
+        if (args.size() <= arity) return visit_app_default(e);
+        /* This `fix_core_n` application is an overapplication.
+           The `fix_core_n` is implemented by the runtime, and the result
+           is a closure. This is bad for performance. We should
+           replace it with `fix_core_m` (if the runtime contains one) */
+        unsigned num_extra = args.size() - arity;
+        unsigned m = n + num_extra;
+        optional<expr> fix_core_m = mk_enf_fix_core(m);
+        if (!fix_core_m) return visit_app_default(e);
+        buffer<expr> new_args;
+        /* Add α_1 ... α_n and β */
+        for (unsigned i = 0; i < m+1; i++) {
+            new_args.push_back(mk_enf_neutral());
+        }
+        new_args.append(args.size() - n - 1, args.data() + n + 1);
+        return mk_app(*fix_core_m, new_args);
+    }
+
     expr visit_app(expr const & e, bool is_let_val) {
         if (is_cases_on_app(env(), e)) {
             return visit_cases(e, is_let_val);
@@ -1406,6 +1445,8 @@ class csimp_fn {
                 return mk_lit(literal(nat(0)));
             } else if (optional<expr> r = try_inline(fn, e, is_let_val)) {
                 return *r;
+            } else if (optional<unsigned> i = is_fix_core(n)) {
+                return visit_fix_core(e, *i);
             } else {
                 return visit_app_default(e);
             }
diff --git a/src/library/compiler/util.cpp b/src/library/compiler/util.cpp
index 4e92dc53df..0e89baa864 100644
--- a/src/library/compiler/util.cpp
+++ b/src/library/compiler/util.cpp
@@ -8,6 +8,7 @@ Author: Leonardo de Moura
 #include <algorithm>
 #include <string>
 #include <limits>
+#include <cctype>
 #include "util/name_hash_set.h"
 #include "kernel/type_checker.h"
 #include "kernel/for_each_fn.h"
@@ -512,6 +513,20 @@ optional<nat> get_num_lit_ext(expr const & e) {
     return to_optional_nat(get_num_lit_core(e.raw()));
 }
 
+optional<unsigned> is_fix_core(name const & n) {
+    if (!n.is_atomic() || !n.is_string()) return optional<unsigned>();
+    string_ref const & r = n.get_string();
+    if (r.length() != 10) return optional<unsigned>();
+    char const * s = r.data();
+    if (std::strncmp(s, "fix_core_", 9) != 0 || !std::isdigit(s[9])) return optional<unsigned>();
+    return optional<unsigned>(s[9] - '0');
+}
+
+optional<expr> mk_enf_fix_core(unsigned n) {
+    if (n == 0 || n > 6) return none_expr();
+    return some_expr(mk_constant(name("fix_core").append_after(n)));
+}
+
 void initialize_compiler_util() {
     g_neutral_expr     = new expr(mk_constant("_neutral"));
     g_unreachable_expr = new expr(mk_constant("_unreachable"));
diff --git a/src/library/compiler/util.h b/src/library/compiler/util.h
index c968aab3f7..482d0628af 100644
--- a/src/library/compiler/util.h
+++ b/src/library/compiler/util.h
@@ -161,6 +161,14 @@ environment register_stage2_decl(environment const & env, name const & n, expr c
 optional<nat> get_num_lit_ext(expr const & e);
 inline bool is_morally_num_lit(expr const & e) { return static_cast<bool>(get_num_lit_ext(e)); }
 
+/* Return `some n` if `c` is of the form `fix_core_n` where `n in [1, 6]`.
+   Remark: this function is assuming the core library contains `fix_core_1` ... `fix_core_6` definitions. */
+optional<unsigned> is_fix_core(name const & c);
+/* Return the `fix_core_n` constant, and `none` if `n` is not in `[1, 6]`.
+   Remark: this function is assuming the core library contains `fix_core_1` ... `fix_core_6` definitions.
+   Remark: this function assumes universe levels have already been erased. */
+optional<expr> mk_enf_fix_core(unsigned n);
+
 void initialize_compiler_util();
 void finalize_compiler_util();
 }