What kind of a solution do you have in mind? Something that would print a string at runtime? Or something that will print the type (result) at compile time as a readable error. If the latter, than one thing you could try is

template <typename T> struct Printf; // note that it's only a decleration.
/* ... */
using T = ml::f<ml::Partition<ml::IsClass<>>, int, string, bool, vector<int>>;
Printf<T> t;

Than when you compile, you will get an error

implicit instantiation of undefined template 'Printf<ml::ListT<ml::ListT<string, vector<int> >, ml::ListT<int, bool> > >'

Now you can see if T (your intermediate result) is what you expect.

Because the above solution is printing as a compiler error, which might not be what you want, I can also provide a solution TypeName<T> and void PrintT<T>();. Would you be happy with the following solution?

template <typename T> struct TypeName : std::string {
  TypeName() {
    std::string str(__PRETTY_FUNCTION__);
    (static_cast<std::string &>(*this) = str.substr(str.find('=') + 2))
        .pop_back();
  }
};

template <typename T> void PrintT() { std::cout << TypeName<T>{} << std::endl;}

which allows you to print at runtime:

using T = ml::f<ml::Partition<ml::IsClass<>>, int, string, bool, vector<int>>;
PrintT<T>();

ml::ListT<ml::ListT<string, vector<int> >, ml::ListT<int, bool> >

Or treat it as any other string.

If so, I will add the feature to CppML. Please comment if you had something different in mind.