refactor(kernel/find_fn): simplify find_fn module

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

src/kernel/find_fn.h

@@ -9,34 +9,19 @@ Author: Leonardo de Moura
 #include "kernel/expr.h"
 
 namespace lean {
-class find_fn {
-    template<typename P>
-    struct pred_fn {
-        optional<expr> & m_result;
-        P                m_p;
-        pred_fn(optional<expr> & result, P const & p):m_result(result), m_p(p) {}
-        bool operator()(expr const & e, unsigned offset) {
-            if (m_result) {
-                return false; // already found result, stop the search
-            } else if (m_p(e, offset)) {
-                m_result = e;
-                return false; // stop the search
-            } else {
-                return true; // continue the search
-            }
-        }
-    };
-    optional<expr> m_result;
-    for_each_fn    m_proc;
-public:
-    template<typename P> find_fn(P const & p):m_proc(pred_fn<P>(m_result, p)) {}
-    optional<expr> operator()(expr const & e) { m_proc(e); return m_result; }
-};
-
-/**
-   \brief Return a subexpression of \c e that satisfies the predicate \c p.
-*/
+/** \brief Return a subexpression of \c e that satisfies the predicate \c p. */
 template<typename P> optional<expr> find(expr const & e, P p) {
-    return find_fn(p)(e);
+    optional<expr> result;
+    for_each(e, [&](expr const & e, unsigned offset) {
+            if (result)  {
+                return false;
+            } else if (p(e, offset)) {
+                result = e;
+                return false;
+            } else {
+                return true;
+            }
+        });
+    return result;
 }
 }