Re: [boost] Boost.Hana - Type Classes, data types and categories (tags).

17 Sep 2014


      Le 17/09/14 22:49, Louis Dionne a écrit :
...
Vicente J. Botet Escriba  writes:
...
Hi Louis,
There is a lot of very good work on your library.
Thank you!
...
I have some comments in relation on how I have implemented Type Classes
in my prototype.
You wrote a type class emulation in C++? Could you please share the link;
I'd like to have a look!
The ideas are similar. The form changes.
https://github.com/ptal/expected
https://github.com/ptal/expected/tree/master/include/boost/functional
...
...
About datatype<T>. The library proposes type classes and data types. The
name datatype_t<> is confusing as the datatype<T> meta-functions must
not return one of the defined data types. I see it much more as a
category, which is used to map the data type to a category used as
parameter of the instantiation
TC::Instance.
IIUC, you're pointing out that `datatype_t` is usually not
the same as `decltype(object)`, which is inconsistent with the way the term
"Data type" is used in Hakell. If so, you are right about the inconsistency.
Regardless of this particular issue, I'll have to rename some stuff in Hana
as others have pointed out. I haven't done it yet since I haven't found names
that are incredibly better than "data type" so far, but I'm adding "category"
to the list. Another candidate is "tag", which would have the additional
benefit of being familiar to Fusion and MPL people.
Why not.
...
...
In addition, I wonder if the category associated shouldn't depend on the
type class we want to map.
TC::Instance>
I'm not sure why that would be useful. Do you have a use case in mind?
After more thoughts the user can always specialize TC::Instance<T> 
inheriting from TC::Instance<CatA>.

struct TC::Instance<T> : TC::Instance<CatA> {};
...
...
About naming: Haskell doesn't have namespaces and all the functions are
at the same scope. Your library makes use of a lot of non-member
functions, and so name clashing must be avoided. What about moving all
the non-member functions of the Type Class to a specific namespace? E.g.
struct Monad { ... };
namespace monad {
      using namespace applicatives;
      ... bind()
    };
The use would need to use the type class namespace explicitly
auto x = monad::bind(m, f)
or introduce the namespace of the type class
using namespace monad;
    auto x = bind(m, f);
I had not thought about the issue because right now, name clashing is avoided
simply by not having two functions with the same name. This has worked well so
far. Since this is the simplest way to do things, I'd rather keep it that way.
Of course, if I need to introduce a function whose name would clash, then I
will strongly consider your suggestion; thank you for that.
You are welcome,
Vicente