Le 17/09/14 22:49, Louis Dionne a écrit :

Vicente J. Botet Escriba writes:

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Hi Louis,

There is a lot of very good work on your library. Thank you!

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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

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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.

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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> {};

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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

Vicente J. Botet Escriba writes:

Le 17/09/14 23:30, Vicente J. Botet Escriba a écrit :

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Le 17/09/14 22:49, Louis Dionne a écrit :

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Vicente J. Botet Escriba writes:

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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> {};

I think I don't understand what you are trying to achieve here. What exactly are CatA and T? What effect do you want to achieve in doing such a type class instantiation (instantiation in Hana/Haskell terms).

A deeper analysis of your code let me think that you have already something similar to what I was requesting.

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The previous specialization let the data type class define directly how is it could be seen as an instance of a given type class NAME. I see however that this has been used only fro the Record type class.

boost/hana/record/macros.hpp: struct hana_Record : ::boost::hana::Record::mcd { \

So here you have the use_case you were requesting.

Given a data type D, it is possible to instantiate any unary type class TC from inside D's definition as follows: struct D { struct hana_TC { // type class definition }; }; This is true for all unary type classes. However, since the instantiation of the Record type class is very straightforward, a macro is provided to make it even easier. Note that the ability to instantiate a type class from inside a data type's definition is not documented right now. Regards, Louis

Niall Douglas writes:

On 17 Sep 2014 at 20:49, Louis Dionne wrote:

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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.

namespaces can be used for disambiguation, but they are actually much more useful and powerful than that. You can map namespaces into other namespaces, mash up namespaces into customised variants and with template aliasing you can now easily metaprogram namespacing too, so you could have the results of a Hana operation cause one set of things to be mapped into some destination namespace as against another set of things. And then, you see, a bind() function can actually have metaprogrammed meaning and implementation which is not only neato, but very intuitive for the end user.

I'm not sure I understand you. I do understand how we can map namespaces into other namespaces: namespace A { } namespace B = A; I also understand how we can mash up namespaces: namespace A { } namespace B { } namespace mash { using namespace A; using namespace B; } But I don't understand how template aliases come into play and give me more flexibility. Would you care to give a basic example of what you meant? Thanks, Louis

Benedek Thaler writes:

On Sat, Sep 20, 2014 at 5:03 PM, Louis Dionne wrote: [...]

I'm not sure what Niall is referring to, but namespaces come into handy when using the swap trick described in effective STL:

template<typename T> library_function(T& a, T& b) { using std::swap; swap(a,b); // will use specialized swap in namespace of T if available }

Yes, ADL can prove itself useful. However, Hana uses tag dispatching like Boost.Fusion and Boost.MPL for customization because it deals with objects of heterogeneous types. Since the actual C++ type of some objects can be impossible or very cumbersome to pattern match, I don't think ADL is the right way to go in our case. Regards, Louis

On 20 Sep 2014 at 15:03, Louis Dionne wrote:

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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.

namespaces can be used for disambiguation, but they are actually much more useful and powerful than that. You can map namespaces into other namespaces, mash up namespaces into customised variants and with template aliasing you can now easily metaprogram namespacing too, so you could have the results of a Hana operation cause one set of things to be mapped into some destination namespace as against another set of things. And then, you see, a bind() function can actually have metaprogrammed meaning and implementation which is not only neato, but very intuitive for the end user.

I'm not sure I understand you. I do understand how we can map namespaces into other namespaces:

namespace A { } namespace B = A;

I also understand how we can mash up namespaces:

namespace A { } namespace B { } namespace mash { using namespace A; using namespace B; }

But I don't understand how template aliases come into play and give me more flexibility. Would you care to give a basic example of what you meant?

The way I look at it is this: templated types are templated namespaces with restrictions on what you can put in them (aside: I have never understood why you can't template namespaces and use namespaces for inheritance, including types and other namespaces), while namespaces let you draw arbitrary lines around segments of API and mux them as appropriate into different default visibility according to point of use. Let us take an example: namespace A { template<class T> using vector = select_vector_impl; } namespace B { template<class T> using vector = select_vector_impl; } namespace C { // I specifically say I use the same vector as namespace B template<class T> using B::vector<T>; } The annoying thing with the present C++ syntax is that for every type item you wish to mash up you need a separate chain of namespace using going into some specialised type which then jumps through metaprogramming to map the type. This is tedious, but I would assume that something like Edward's VMD library ought to allow automation of the generation of these chains such that one could then simply write: namespace A { USING_STL_IMPL(A); // unordered_map => std::unordered_map etc } namespace B { USING_STL_IMPL(B); // unordered_map => boost::unordered_map etc } This might not look like it conveys much utility on its own. But imagine if you were handed a large lump of library code and you are not allowed to touch or modify it, but it has several bugs and problems, and your client code needs to work around them. If the original library hard codes that namespace A uses the STL and namespace B uses boost, your hands are tied and you probably will have to write a clang AST parser to bind the original library into a duplicate, but fixed copy of its namespace. If instead tag based symbol binding is used, you can partially specialise the template programming sitting in between the namespace bind and its implementation, and thus your library becomes much more easier for others to mash up and mess around with from the outside. All this said, is it worth the programmer manually doing these binds when a clang AST tool could grok the original and generate a metaprogrammed bound but otherwise identical duplicate? That I currently don't know the answer to. If a clang AST tool can do so, forget I ever wrote this email or raised this topic, as it is moot. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/