diff --git a/README.md b/README.md
index db61f643b..16e959d56 100644
--- a/README.md
+++ b/README.md
@@ -261,6 +261,7 @@ Before parsing, you can set the following options:
 -   `->description(str)`: Set/change the description.
 -   `->multi_option_policy(CLI::MultiOptionPolicy::Throw)`: Set the multi-option policy. Shortcuts available: `->take_last()`, `->take_first()`, and `->join()`. This will only affect options expecting 1 argument or bool flags (which do not inherit their default but always start with a specific policy).
 -   `->check(CLI::IsMember(...))`: Require an option be a member of a given set. See below for options.
+-   `->transform(CLI::IsMember(...))`: Require an option be a member of a given set or map. Can change the parse. See below for options.
 -   `->check(CLI::ExistingFile)`: Requires that the file exists if given.
 -   `->check(CLI::ExistingDirectory)`: Requires that the directory exists.
 -   `->check(CLI::ExistingPath)`: Requires that the path (file or directory) exists.
@@ -283,7 +284,9 @@ of `IsMember`:
 -   `CLI::IsMember({"choice1", "choice2"})`: Select from exact match to choices.
 -   `CLI::IsMember({"choice1", "choice2"}, CLI::ignore_case, CLI::ignore_underscore)`: Match things like `Choice_1`, too.
 -   `CLI::IsMember(std::set<int>({2,3,4}))`: Most containers and types work; you just need `std::begin`, `std::end`, and `::value_type`.
+-   `CLI::IsMember(std::map<std::string, int>({{"one", 1}, {"two", 2}}))`: You can use maps; in `->transform()` these replace the matched value with the key.
 -   `auto p = std::make_shared<std::vector<std::string>>(std::initializer_list<std::string>("one", "two")); CLI::IsMember(p)`: You can modify `p` later.
+-   Note that you can combine validators with `|`, and only the matched validation will modify the output in transform. The first `IsMember` is the only one that will print in help, though the error message will include all Validators.
 
 On the command line, options can be given as:
 
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index ba303b241..01590c319 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -130,7 +130,7 @@ add_cli_exe(enum enum.cpp)
 add_test(NAME enum_pass COMMAND enum -l 1)
 add_test(NAME enum_fail COMMAND enum -l 4)
 set_property(TEST enum_fail PROPERTY PASS_REGULAR_EXPRESSION
-    "--level: 4 not in {0,1,2}")
+    "--level: 4 not in {High,Medium,Low} | 4 not in {0,1,2}")
 
 add_cli_exe(modhelp modhelp.cpp)
 add_test(NAME modhelp COMMAND modhelp -a test -h)
diff --git a/examples/enum.cpp b/examples/enum.cpp
index 68c78f63c..19b92fbde 100644
--- a/examples/enum.cpp
+++ b/examples/enum.cpp
@@ -1,26 +1,25 @@
 #include <CLI/CLI.hpp>
+#include <map>
 #include <sstream>
 
 enum class Level : int { High, Medium, Low };
 
-std::istream &operator>>(std::istream &in, Level &level) {
-    int i;
-    in >> i;
-    level = static_cast<Level>(i);
-    return in;
-}
-
-std::ostream &operator<<(std::ostream &in, const Level &level) { return in << static_cast<int>(level); }
-
 int main(int argc, char **argv) {
     CLI::App app;
 
+    std::map<std::string, Level> map = {{"High", Level::High}, {"Medium", Level::Medium}, {"Low", Level::Low}};
+
     Level level;
+
     app.add_option("-l,--level", level, "Level settings")
-        ->check(CLI::IsMember({Level::High, Level::Medium, Level::Low}))
-        ->type_name("enum/Level in {High=0, Medium=1, Low=2}");
+        ->required()
+        ->transform(CLI::IsMember(map, CLI::ignore_case) | CLI::IsMember({Level::High, Level::Medium, Level::Low}));
 
     CLI11_PARSE(app, argc, argv);
 
+    // CLI11's built in enum streaming can be used outside CLI11 like this:
+    using namespace CLI::enums;
+    std::cout << "Enum received: " << level << std::endl;
+
     return 0;
 }
diff --git a/include/CLI/App.hpp b/include/CLI/App.hpp
index 6ab804e89..d1aee0e98 100644
--- a/include/CLI/App.hpp
+++ b/include/CLI/App.hpp
@@ -695,32 +695,32 @@ class App {
     }
 
     /// Add set of options, string only, ignore case (no default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_case)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_case)) instead")
     Option *add_set_ignore_case(std::string option_name,
                                 std::string &member,           ///< The selected member of the set
                                 std::set<std::string> options, ///< The set of possibilities
                                 std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{options, CLI::ignore_case});
+        opt->transform(IsMember{options, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore case (no default, set can be changed afterwards - do not destroy the
     /// set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_case)) with a (shared) pointer instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_case)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_case(std::string option_name,
                                         std::string &member,                  ///< The selected member of the set
                                         const std::set<std::string> &options, ///< The set of possibilities
                                         std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{&options, CLI::ignore_case});
+        opt->transform(IsMember{&options, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore case (default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_case)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_case)) instead")
     Option *add_set_ignore_case(std::string option_name,
                                 std::string &member,           ///< The selected member of the set
                                 std::set<std::string> options, ///< The set of possibilities
@@ -728,13 +728,13 @@ class App {
                                 bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{options, CLI::ignore_case});
+        opt->transform(IsMember{options, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore case (default, set can be changed afterwards - do not destroy the set)
     /// DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(...)) with a (shared) pointer instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(...)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_case(std::string option_name,
                                         std::string &member,                  ///< The selected member of the set
                                         const std::set<std::string> &options, ///< The set of possibilities
@@ -742,37 +742,37 @@ class App {
                                         bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{&options, CLI::ignore_case});
+        opt->transform(IsMember{&options, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore (no default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_underscore)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_underscore)) instead")
     Option *add_set_ignore_underscore(std::string option_name,
                                       std::string &member,           ///< The selected member of the set
                                       std::set<std::string> options, ///< The set of possibilities
                                       std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{options, CLI::ignore_underscore});
+        opt->transform(IsMember{options, CLI::ignore_underscore});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore (no default, set can be changed afterwards - do not destroy
     /// the set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_underscore)) with a (shared) pointer instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_underscore)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_underscore(std::string option_name,
                                               std::string &member,                  ///< The selected member of the set
                                               const std::set<std::string> &options, ///< The set of possibilities
                                               std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{options, CLI::ignore_underscore});
+        opt->transform(IsMember{options, CLI::ignore_underscore});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore (default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_underscore)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_underscore)) instead")
     Option *add_set_ignore_underscore(std::string option_name,
                                       std::string &member,           ///< The selected member of the set
                                       std::set<std::string> options, ///< The set of possibilities
@@ -780,13 +780,13 @@ class App {
                                       bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{options, CLI::ignore_underscore});
+        opt->transform(IsMember{options, CLI::ignore_underscore});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore (default, set can be changed afterwards - do not destroy the
     /// set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_underscore)) with a (shared) pointer instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_underscore)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_underscore(std::string option_name,
                                               std::string &member,                  ///< The selected member of the set
                                               const std::set<std::string> &options, ///< The set of possibilities
@@ -794,38 +794,38 @@ class App {
                                               bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{&options, CLI::ignore_underscore});
+        opt->transform(IsMember{&options, CLI::ignore_underscore});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore and case (no default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) instead")
     Option *add_set_ignore_case_underscore(std::string option_name,
                                            std::string &member,           ///< The selected member of the set
                                            std::set<std::string> options, ///< The set of possibilities
                                            std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{options, CLI::ignore_underscore, CLI::ignore_case});
+        opt->transform(IsMember{options, CLI::ignore_underscore, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore and case (no default, set can be changed afterwards - do not
     /// destroy the set) DEPRECATED
     CLI11_DEPRECATED(
-        "Use ->check(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) with a (shared) pointer instead")
+        "Use ->transform(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_case_underscore(std::string option_name,
                                                    std::string &member, ///< The selected member of the set
                                                    const std::set<std::string> &options, ///< The set of possibilities
                                                    std::string description = "") {
 
         Option *opt = add_option(option_name, member, std::move(description));
-        opt->check(IsMember{&options, CLI::ignore_underscore, CLI::ignore_case});
+        opt->transform(IsMember{&options, CLI::ignore_underscore, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore and case (default, static set) DEPRECATED
-    CLI11_DEPRECATED("Use ->check(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) instead")
+    CLI11_DEPRECATED("Use ->transform(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) instead")
     Option *add_set_ignore_case_underscore(std::string option_name,
                                            std::string &member,           ///< The selected member of the set
                                            std::set<std::string> options, ///< The set of possibilities
@@ -833,14 +833,14 @@ class App {
                                            bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{options, CLI::ignore_underscore, CLI::ignore_case});
+        opt->transform(IsMember{options, CLI::ignore_underscore, CLI::ignore_case});
         return opt;
     }
 
     /// Add set of options, string only, ignore underscore and case (default, set can be changed afterwards - do not
     /// destroy the set) DEPRECATED
     CLI11_DEPRECATED(
-        "Use ->check(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) with a (shared) pointer instead")
+        "Use ->transform(CLI::IsMember(..., CLI::ignore_case, CLI::ignore_underscore)) with a (shared) pointer instead")
     Option *add_mutable_set_ignore_case_underscore(std::string option_name,
                                                    std::string &member, ///< The selected member of the set
                                                    const std::set<std::string> &options, ///< The set of possibilities
@@ -848,7 +848,7 @@ class App {
                                                    bool defaulted) {
 
         Option *opt = add_option(option_name, member, std::move(description), defaulted);
-        opt->check(IsMember{&options, CLI::ignore_underscore, CLI::ignore_case});
+        opt->transform(IsMember{&options, CLI::ignore_underscore, CLI::ignore_case});
         return opt;
     }
 
diff --git a/include/CLI/Option.hpp b/include/CLI/Option.hpp
index 4c57af83b..9a0a9f865 100644
--- a/include/CLI/Option.hpp
+++ b/include/CLI/Option.hpp
@@ -303,7 +303,12 @@ class Option : public OptionBase<Option> {
 
     /// Adds a validator with a built in type name
     Option *check(const Validator &validator) {
-        validators_.emplace_back(validator.func);
+        std::function<std::string(std::string &)> func = validator.func;
+        validators_.emplace_back([func](const std::string &value) {
+            /// Throw away changes to the string value
+            std::string ignore_changes_value = value;
+            return func(ignore_changes_value);
+        });
         if(validator.tname_function)
             type_name_fn(validator.tname_function);
         else if(!validator.tname.empty())
@@ -317,6 +322,16 @@ class Option : public OptionBase<Option> {
         return this;
     }
 
+    /// Adds a transforming validator with a built in type name
+    Option *transform(const Validator &validator) {
+        validators_.emplace_back(validator.func);
+        if(validator.tname_function)
+            type_name_fn(validator.tname_function);
+        else if(!validator.tname.empty())
+            type_name(validator.tname);
+        return this;
+    }
+
     /// Adds a validator-like function that can change result
     Option *transform(std::function<std::string(std::string)> func) {
         validators_.emplace_back([func](std::string &inout) {
diff --git a/include/CLI/StringTools.hpp b/include/CLI/StringTools.hpp
index 92edc47a7..499295666 100644
--- a/include/CLI/StringTools.hpp
+++ b/include/CLI/StringTools.hpp
@@ -13,6 +13,31 @@
 #include <vector>
 
 namespace CLI {
+
+/// Include the items in this namespace to get free conversion of enums to/from streams.
+/// (This is available inside CLI as well, so CLI11 will use this without a using statement).
+namespace enums {
+
+/// output streaming for enumerations
+template <typename T, typename = typename std::enable_if<std::is_enum<T>::value>::type>
+std::ostream &operator<<(std::ostream &in, const T &item) {
+    // make sure this is out of the detail namespace otherwise it won't be found when needed
+    return in << static_cast<typename std::underlying_type<T>::type>(item);
+}
+
+/// input streaming for enumerations
+template <typename T, typename = typename std::enable_if<std::is_enum<T>::value>::type>
+std::istream &operator>>(std::istream &in, T &item) {
+    typename std::underlying_type<T>::type i;
+    in >> i;
+    item = static_cast<T>(i);
+    return in;
+}
+} // namespace enums
+
+/// Export to CLI namespace
+using namespace enums;
+
 namespace detail {
 
 // Based on http://stackoverflow.com/questions/236129/split-a-string-in-c
diff --git a/include/CLI/TypeTools.hpp b/include/CLI/TypeTools.hpp
index 7eafbbae7..6904eb2ae 100644
--- a/include/CLI/TypeTools.hpp
+++ b/include/CLI/TypeTools.hpp
@@ -28,9 +28,17 @@ constexpr enabler dummy = {};
 /// We could check to see if C++14 is being used, but it does not hurt to redefine this
 /// (even Google does this: https://github.com/google/skia/blob/master/include/private/SkTLogic.h)
 /// It is not in the std namespace anyway, so no harm done.
-
 template <bool B, class T = void> using enable_if_t = typename std::enable_if<B, T>::type;
 
+/// A copy of std::void_t from C++17 (helper for C++11 and C++14)
+template <typename... Ts> struct make_void { using type = void; };
+
+/// A copy of std::void_t from C++17 - same reasoning as enable_if_t, it does not hurt to redefine
+template <typename... Ts> using void_t = typename make_void<Ts...>::type;
+
+/// A copy of std::conditional_t from C++14 - same reasoning as enable_if_t, it does not hurt to redefine
+template <bool B, class T, class F> using conditional_t = typename std::conditional<B, T, F>::type;
+
 /// Check to see if something is a vector (fail check by default)
 template <typename T> struct is_vector : std::false_type {};
 
@@ -54,6 +62,16 @@ template <typename T> struct is_copyable_ptr {
     static bool const value = is_shared_ptr<T>::value || std::is_pointer<T>::value;
 };
 
+/// This can be specialized to override the type deduction for IsMember.
+template <typename T> struct IsMemberType { using type = T; };
+
+/// The main custom type needed here is const char * should be a string.
+template <> struct IsMemberType<const char *> { using type = std::string; };
+
+namespace detail {
+
+// These are utilites for IsMember
+
 /// Handy helper to access the element_type generically. This is not part of is_copyable_ptr because it requires that
 /// pointer_traits<T> be valid.
 template <typename T> struct element_type {
@@ -65,13 +83,34 @@ template <typename T> struct element_type {
 /// the container
 template <typename T> struct element_value_type { using type = typename element_type<T>::type::value_type; };
 
-/// This can be specialized to override the type deduction for IsMember.
-template <typename T> struct IsMemberType { using type = T; };
+/// Adaptor for map-like structure: This just wraps a normal container in a few utilities that do almost nothing.
+template <typename T, typename _ = void> struct pair_adaptor : std::false_type {
+    using value_type = typename T::value_type;
+    using first_type = typename std::remove_const<value_type>::type;
+    using second_type = typename std::remove_const<value_type>::type;
 
-/// The main custom type needed here is const char * should be a string.
-template <> struct IsMemberType<const char *> { using type = std::string; };
+    /// Get the first value (really just the underlying value)
+    template <typename Q> static first_type first(Q &&value) { return value; }
+    /// Get the second value (really just the underlying value)
+    template <typename Q> static second_type second(Q &&value) { return value; }
+};
 
-namespace detail {
+/// Adaptor for map-like structure (true version, must have key_type and mapped_type).
+/// This wraps a mapped container in a few utilities access it in a general way.
+template <typename T>
+struct pair_adaptor<
+    T,
+    conditional_t<false, void_t<typename T::value_type::first_type, typename T::value_type::second_type>, void>>
+    : std::true_type {
+    using value_type = typename T::value_type;
+    using first_type = typename std::remove_const<typename value_type::first_type>::type;
+    using second_type = typename std::remove_const<typename value_type::second_type>::type;
+
+    /// Get the first value (really just the underlying value)
+    template <typename Q> static first_type first(Q &&value) { return value.first; }
+    /// Get the second value (really just the underlying value)
+    template <typename Q> static second_type second(Q &&value) { return value.second; }
+};
 
 // Type name print
 
diff --git a/include/CLI/Validators.hpp b/include/CLI/Validators.hpp
index 5d4146ed4..e050669fe 100644
--- a/include/CLI/Validators.hpp
+++ b/include/CLI/Validators.hpp
@@ -57,42 +57,46 @@ class Validator {
         return func(value);
     };
 
-    /// Combining validators is a new validator
+    /// Combining validators is a new validator. Type comes from left validator if function, otherwise only set if the
+    /// same.
     Validator operator&(const Validator &other) const {
         Validator newval;
         newval.tname = (tname == other.tname ? tname : "");
+        newval.tname_function = tname_function;
 
         // Give references (will make a copy in lambda function)
         const std::function<std::string(std::string & filename)> &f1 = func;
         const std::function<std::string(std::string & filename)> &f2 = other.func;
 
-        newval.func = [f1, f2](std::string &filename) {
-            std::string s1 = f1(filename);
-            std::string s2 = f2(filename);
+        newval.func = [f1, f2](std::string &input) {
+            std::string s1 = f1(input);
+            std::string s2 = f2(input);
             if(!s1.empty() && !s2.empty())
-                return s1 + " & " + s2;
+                return s1 + " AND " + s2;
             else
                 return s1 + s2;
         };
         return newval;
     }
 
-    /// Combining validators is a new validator
+    /// Combining validators is a new validator. Type comes from left validator if function, otherwise only set if the
+    /// same.
     Validator operator|(const Validator &other) const {
         Validator newval;
         newval.tname = (tname == other.tname ? tname : "");
+        newval.tname_function = tname_function;
 
         // Give references (will make a copy in lambda function)
-        const std::function<std::string(std::string & filename)> &f1 = func;
-        const std::function<std::string(std::string & filename)> &f2 = other.func;
+        const std::function<std::string(std::string &)> &f1 = func;
+        const std::function<std::string(std::string &)> &f2 = other.func;
 
-        newval.func = [f1, f2](std::string &filename) {
-            std::string s1 = f1(filename);
-            std::string s2 = f2(filename);
+        newval.func = [f1, f2](std::string &input) {
+            std::string s1 = f1(input);
+            std::string s2 = f2(input);
             if(s1.empty() || s2.empty())
                 return std::string();
             else
-                return s1 + " & " + s2;
+                return s1 + " OR " + s2;
         };
         return newval;
     }
@@ -274,6 +278,7 @@ auto smart_deref(T value) -> decltype(*value) {
 template <typename T, enable_if_t<!is_copyable_ptr<T>::value, detail::enabler> = detail::dummy> T smart_deref(T value) {
     return value;
 }
+
 } // namespace detail
 
 /// Verify items are in a set
@@ -287,18 +292,19 @@ class IsMember : public Validator {
         : IsMember(std::vector<T>(values), std::forward<Args>(args)...) {}
 
     /// This checks to see if an item is in a set (empty function)
-    template <typename T>
-    explicit IsMember(T set)
-        : IsMember(std::move(set),
-                   std::function<typename IsMemberType<typename element_value_type<T>::type>::type(
-                       typename IsMemberType<typename element_value_type<T>::type>::type)>()) {}
+    template <typename T> explicit IsMember(T set) : IsMember(std::move(set), nullptr) {}
 
     /// This checks to see if an item is in a set: pointer or copy version. You can pass in a function that will filter
     /// both sides of the comparison before computing the comparison.
     template <typename T, typename F> explicit IsMember(T set, F filter_function) {
+
         // Get the type of the contained item - requires a container have ::value_type
-        using item_t = typename element_value_type<T>::type;
-        using local_item_t = typename IsMemberType<item_t>::type;
+        // if the type does not have first_type and second_type, these are both value_type
+        using element_t = typename detail::element_type<T>::type;            // Removes (smart) pointers if needed
+        using item_t = typename detail::pair_adaptor<element_t>::first_type; // Is value_type if not a map
+
+        using local_item_t = typename IsMemberType<item_t>::type; // This will convert bad types to good ones
+                                                                  // (const char * to std::string)
 
         // Make a local copy of the filter function, using a std::function if not one already
         std::function<local_item_t(local_item_t)> filter_fn = filter_function;
@@ -306,15 +312,19 @@ class IsMember : public Validator {
         // This is the type name for help, it will take the current version of the set contents
         tname_function = [set]() {
             std::stringstream out;
-            out << detail::type_name<item_t>() << " in {" << detail::join(detail::smart_deref(set), ",") << "}";
+            out << "{";
+            int i = 0; // I don't like counters like this
+            for(const auto &v : detail::smart_deref(set))
+                out << (i++ == 0 ? "" : ",") << detail::pair_adaptor<element_t>::first(v);
+            out << "}";
             return out.str();
         };
 
         // This is the function that validates
         // It stores a copy of the set pointer-like, so shared_ptr will stay alive
         func = [set, filter_fn](std::string &input) {
-            for(const item_t &v : detail::smart_deref(set)) {
-                local_item_t a = v;
+            for(const auto &v : detail::smart_deref(set)) {
+                local_item_t a = detail::pair_adaptor<element_t>::first(v);
                 local_item_t b;
                 if(!detail::lexical_cast(input, b))
                     throw ValidationError(input); // name is added later
@@ -328,9 +338,10 @@ class IsMember : public Validator {
                 if(a == b) {
                     // Make sure the version in the input string is identical to the one in the set
                     // Requires std::stringstream << be supported on T.
-                    if(filter_fn) {
+                    // If this is a map, output the map instead.
+                    if(filter_fn || detail::pair_adaptor<element_t>::value) {
                         std::stringstream out;
-                        out << v;
+                        out << detail::pair_adaptor<element_t>::second(v);
                         input = out.str();
                     }
 
@@ -340,11 +351,17 @@ class IsMember : public Validator {
             }
 
             // If you reach this point, the result was not found
-            return input + " not in {" + detail::join(detail::smart_deref(set), ",") + "}";
+            std::stringstream out;
+            out << input << " not in {";
+            int i = 0; // I still don't like counters like this
+            for(const auto &v : detail::smart_deref(set))
+                out << (i++ == 0 ? "" : ",") << detail::pair_adaptor<element_t>::first(v);
+            out << "}";
+            return out.str();
         };
     }
 
-    /// You can pass in as many filter functions as you like, they nest
+    /// You can pass in as many filter functions as you like, they nest (string only currently)
     template <typename T, typename... Args>
     IsMember(T set, filter_fn_t filter_fn_1, filter_fn_t filter_fn_2, Args &&... other)
         : IsMember(std::move(set),
diff --git a/tests/HelpTest.cpp b/tests/HelpTest.cpp
index 1a48d73fe..4656a4135 100644
--- a/tests/HelpTest.cpp
+++ b/tests/HelpTest.cpp
@@ -237,6 +237,27 @@ TEST(THelp, ManualSetters) {
     EXPECT_THAT(help, HasSubstr("=14"));
 }
 
+TEST(THelp, ManualSetterOverFunction) {
+
+    CLI::App app{"My prog"};
+
+    int x = 1;
+
+    CLI::Option *op1 = app.add_option("--op1", x)->check(CLI::IsMember({1, 2}));
+    CLI::Option *op2 = app.add_option("--op2", x)->transform(CLI::IsMember({1, 2}));
+    op1->default_str("12");
+    op1->type_name("BIGGLES");
+    op2->type_name("QUIGGLES");
+    EXPECT_EQ(x, 1);
+
+    std::string help = app.help();
+
+    EXPECT_THAT(help, HasSubstr("=12"));
+    EXPECT_THAT(help, HasSubstr("BIGGLES"));
+    EXPECT_THAT(help, HasSubstr("QUIGGLES"));
+    EXPECT_THAT(help, Not(HasSubstr("1,2")));
+}
+
 TEST(THelp, Subcom) {
     CLI::App app{"My prog"};
 
@@ -789,6 +810,19 @@ TEST(THelp, CombinedValidatorsPathyText) {
     EXPECT_THAT(help, HasSubstr("PATH"));
 }
 
+// Don't do this in real life, please (and transform does nothing here)
+TEST(THelp, CombinedValidatorsPathyTextAsTransform) {
+    CLI::App app;
+
+    std::string filename;
+    app.add_option("--f1", filename)->transform(CLI::ExistingPath | CLI::NonexistentPath);
+
+    // Combining validators with the same type string is OK
+    std::string help = app.help();
+    EXPECT_THAT(help, Not(HasSubstr("TEXT")));
+    EXPECT_THAT(help, HasSubstr("PATH"));
+}
+
 // #113 Part 2
 TEST(THelp, ChangingSet) {
     CLI::App app;
diff --git a/tests/SetTest.cpp b/tests/SetTest.cpp
index 2db748ecb..079c821e0 100644
--- a/tests/SetTest.cpp
+++ b/tests/SetTest.cpp
@@ -1,4 +1,5 @@
 #include "app_helper.hpp"
+#include <map>
 
 static_assert(CLI::is_shared_ptr<std::shared_ptr<int>>::value == true, "is_shared_ptr should work on shared pointers");
 static_assert(CLI::is_shared_ptr<int *>::value == false, "is_shared_ptr should work on pointers");
@@ -9,6 +10,82 @@ static_assert(CLI::is_copyable_ptr<std::shared_ptr<int>>::value == true,
 static_assert(CLI::is_copyable_ptr<int *>::value == true, "is_copyable_ptr should work on pointers");
 static_assert(CLI::is_copyable_ptr<int>::value == false, "is_copyable_ptr should work on non-pointers");
 
+static_assert(CLI::detail::pair_adaptor<std::set<int>>::value == false, "Should not have pairs");
+static_assert(CLI::detail::pair_adaptor<std::map<int, int>>::value == true, "Should have pairs");
+static_assert(CLI::detail::pair_adaptor<std::vector<std::pair<int, int>>>::value == true, "Should have pairs");
+
+TEST_F(TApp, SimpleMaps) {
+    int value;
+    std::map<std::string, int> map = {{"one", 1}, {"two", 2}};
+    auto opt = app.add_option("-s,--set", value)->transform(CLI::IsMember(map));
+    args = {"-s", "one"};
+    run();
+    EXPECT_EQ(1u, app.count("-s"));
+    EXPECT_EQ(1u, app.count("--set"));
+    EXPECT_EQ(1u, opt->count());
+    EXPECT_EQ(value, 1);
+}
+
+TEST_F(TApp, StringStringMap) {
+    std::string value;
+    std::map<std::string, std::string> map = {{"a", "b"}, {"b", "c"}};
+    app.add_option("-s,--set", value)->transform(CLI::IsMember(map));
+    args = {"-s", "a"};
+    run();
+    EXPECT_EQ(value, "b");
+
+    args = {"-s", "b"};
+    run();
+    EXPECT_EQ(value, "c");
+
+    args = {"-s", "c"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+TEST_F(TApp, StringStringMapNoModify) {
+    std::string value;
+    std::map<std::string, std::string> map = {{"a", "b"}, {"b", "c"}};
+    app.add_option("-s,--set", value)->check(CLI::IsMember(map));
+    args = {"-s", "a"};
+    run();
+    EXPECT_EQ(value, "a");
+
+    args = {"-s", "b"};
+    run();
+    EXPECT_EQ(value, "b");
+
+    args = {"-s", "c"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+enum SimpleEnum { SE_one = 1, SE_two = 2 };
+
+TEST_F(TApp, EnumMap) {
+    SimpleEnum value;
+    std::map<std::string, SimpleEnum> map = {{"one", SE_one}, {"two", SE_two}};
+    auto opt = app.add_option("-s,--set", value)->transform(CLI::IsMember(map));
+    args = {"-s", "one"};
+    run();
+    EXPECT_EQ(1u, app.count("-s"));
+    EXPECT_EQ(1u, app.count("--set"));
+    EXPECT_EQ(1u, opt->count());
+    EXPECT_EQ(value, SE_one);
+}
+
+enum class SimpleEnumC { one = 1, two = 2 };
+
+TEST_F(TApp, EnumCMap) {
+    SimpleEnumC value;
+    std::map<std::string, SimpleEnumC> map = {{"one", SimpleEnumC::one}, {"two", SimpleEnumC::two}};
+    auto opt = app.add_option("-s,--set", value)->transform(CLI::IsMember(map));
+    args = {"-s", "one"};
+    run();
+    EXPECT_EQ(1u, app.count("-s"));
+    EXPECT_EQ(1u, app.count("--set"));
+    EXPECT_EQ(1u, opt->count());
+    EXPECT_EQ(value, SimpleEnumC::one);
+}
+
 TEST_F(TApp, SimpleSets) {
     std::string value;
     auto opt = app.add_option("-s,--set", value)->check(CLI::IsMember{std::set<std::string>({"one", "two", "three"})});
@@ -51,7 +128,7 @@ TEST_F(TApp, SimiShortcutSets) {
     EXPECT_EQ(value, "one");
 
     std::string value2;
-    auto opt2 = app.add_option("--set2", value2)->check(CLI::IsMember({"One", "two", "three"}, CLI::ignore_case));
+    auto opt2 = app.add_option("--set2", value2)->transform(CLI::IsMember({"One", "two", "three"}, CLI::ignore_case));
     args = {"--set2", "onE"};
     run();
     EXPECT_EQ(1u, app.count("--set2"));
@@ -60,7 +137,7 @@ TEST_F(TApp, SimiShortcutSets) {
 
     std::string value3;
     auto opt3 = app.add_option("--set3", value3)
-                    ->check(CLI::IsMember({"O_ne", "two", "three"}, CLI::ignore_case, CLI::ignore_underscore));
+                    ->transform(CLI::IsMember({"O_ne", "two", "three"}, CLI::ignore_case, CLI::ignore_underscore));
     args = {"--set3", "onE"};
     run();
     EXPECT_EQ(1u, app.count("--set3"));
@@ -95,7 +172,7 @@ TEST_F(TApp, OtherTypeSets) {
     EXPECT_THROW(run(), CLI::ValidationError);
 
     std::vector<int> set2 = {-2, 3, 4};
-    auto opt2 = app.add_option("--set2", value)->check(CLI::IsMember(set2, [](int x) { return std::abs(x); }));
+    auto opt2 = app.add_option("--set2", value)->transform(CLI::IsMember(set2, [](int x) { return std::abs(x); }));
     args = {"--set2", "-3"};
     run();
     EXPECT_EQ(1u, app.count("--set2"));
@@ -189,7 +266,7 @@ TEST_F(TApp, InSetWithDefault) {
 TEST_F(TApp, InCaselessSetWithDefault) {
 
     std::string choice = "one";
-    app.add_option("-q,--quick", choice, "", true)->check(CLI::IsMember({"one", "two", "three"}, CLI::ignore_case));
+    app.add_option("-q,--quick", choice, "", true)->transform(CLI::IsMember({"one", "two", "three"}, CLI::ignore_case));
 
     run();
     EXPECT_EQ("one", choice);
@@ -263,7 +340,7 @@ TEST_F(TApp, FailMutableSet) {
 TEST_F(TApp, InSetIgnoreCase) {
 
     std::string choice;
-    app.add_option("-q,--quick", choice)->check(CLI::IsMember({"one", "Two", "THREE"}, CLI::ignore_case));
+    app.add_option("-q,--quick", choice)->transform(CLI::IsMember({"one", "Two", "THREE"}, CLI::ignore_case));
 
     args = {"--quick", "One"};
     run();
@@ -288,7 +365,7 @@ TEST_F(TApp, InSetIgnoreCaseMutableValue) {
 
     std::set<std::string> options{"one", "Two", "THREE"};
     std::string choice;
-    app.add_option("-q,--quick", choice)->check(CLI::IsMember(&options, CLI::ignore_case));
+    app.add_option("-q,--quick", choice)->transform(CLI::IsMember(&options, CLI::ignore_case));
 
     args = {"--quick", "One"};
     run();
@@ -311,7 +388,7 @@ TEST_F(TApp, InSetIgnoreCasePointer) {
 
     std::set<std::string> *options = new std::set<std::string>{"one", "Two", "THREE"};
     std::string choice;
-    app.add_option("-q,--quick", choice)->check(CLI::IsMember(*options, CLI::ignore_case));
+    app.add_option("-q,--quick", choice)->transform(CLI::IsMember(*options, CLI::ignore_case));
 
     args = {"--quick", "One"};
     run();
@@ -341,7 +418,7 @@ TEST_F(TApp, InSetIgnoreUnderscore) {
 
     std::string choice;
     app.add_option("-q,--quick", choice)
-        ->check(CLI::IsMember({"option_one", "option_two", "optionthree"}, CLI::ignore_underscore));
+        ->transform(CLI::IsMember({"option_one", "option_two", "optionthree"}, CLI::ignore_underscore));
 
     args = {"--quick", "option_one"};
     run();
@@ -366,7 +443,8 @@ TEST_F(TApp, InSetIgnoreCaseUnderscore) {
 
     std::string choice;
     app.add_option("-q,--quick", choice)
-        ->check(CLI::IsMember({"Option_One", "option_two", "OptionThree"}, CLI::ignore_case, CLI::ignore_underscore));
+        ->transform(
+            CLI::IsMember({"Option_One", "option_two", "OptionThree"}, CLI::ignore_case, CLI::ignore_underscore));
 
     args = {"--quick", "option_one"};
     run();
@@ -423,8 +501,8 @@ TEST_F(TApp, AddRemoveSetItemsNoCase) {
     std::set<std::string> items{"TYPE1", "TYPE2", "TYPE3", "TYPE4", "TYPE5"};
 
     std::string type1, type2;
-    app.add_option("--type1", type1)->check(CLI::IsMember(&items, CLI::ignore_case));
-    app.add_option("--type2", type2, "", true)->check(CLI::IsMember(&items, CLI::ignore_case));
+    app.add_option("--type1", type1)->transform(CLI::IsMember(&items, CLI::ignore_case));
+    app.add_option("--type2", type2, "", true)->transform(CLI::IsMember(&items, CLI::ignore_case));
 
     args = {"--type1", "TYPe1", "--type2", "TyPE2"};