Re: [Felix-language] [felix] stumped

[john s. <sk...@us...>]

On 25/04/2015, at 3:44 AM, Shayne Fletcher wrote:

> 
> On Fri, Apr 24, 2015 at 1:07 PM, john skaller <sk...@us...> wrote:
> fun ravel[u1,u2,r1,r2] (f1:u1->r1,f2:u2->r2) : u1 * u2 -> r1 * r2
> 
> ​Neat function.

But I want

	ravel[D1,C1,D2,C2,D3,C3,....D9,C9] (f1:D1->D2, f2:D2->C2 ... f9:D9->C9): D1 * D2 *..D9 -> C1 *C2 .. C9;

I can do

	str[D1,D2, ... D9] (p1:D1, p2:D2,.. p9:D9):string 

given str:T->string which should be harder, since it involves polymorphic recursion.

However the latter is just a string fold of the individual components. 
However I don't think Ocaml can do this using any method,
and I'm not sure Haskell can either (in fact Haskell doesn't
even HAVE tuples AFAIK)

Felix and C++ can do it.

However I can't generalise it, even if you want a bool fold of some property,
i.e. fix the return type.

The ravel doesn't need any typeclass, it can be done with a purely
parametric function, but I can't figure it out :)

In Felix and C++ you have to use template instantiation to do 
polymorphic recursion over a typeclass, you cannot do it 
without a typeclass, and I see no way to do it in Felix at all.

In fact, short of making "ravel" a compiler intrinsic, I see no way 
to do it. If I am having to add a compiler intrinsic or two, I would
want to add the most general ones.

Hence reading category theory papers to see what the most
general ones are.

--
john skaller
sk...@us...
http://felix-lang.org