Local evidence and type class instances

[Antoine Latter]

On Sat, Oct 16, 2010 at 11:41 AM, Max Bolingbroke
<batterseapower at hotmail.com> wrote:
>
> On 16 October 2010 15:47, Antoine Latter <aslatter at gmail.com> wrote:
>> Substituting f for f' in this example will change the meaning of the
>> operation significantly, which is, in my mind, hard to explain and
>> reason about.
>
> That's the behaviour I had in mind. There is no doubt that local
> instances are a big complexity increase, but IMHO this behaviour is
> relatively easy to reason about as long as you know about the
> dictionary passing translation. Indeed, it's practically mandatory to
> use dictionary-passing to implement this behaviour. Local instances
> would probably never get added to a compiler (like JHC) that doesn't
> use that technique.
>

>From the point of view of the down-stream engineer, I feel that first
class modules with a module algebra like OCaml would be a lot less
mysterious. Once a thing is locally scoped, it's nicer if I can name
it and abstract over it in a concrete and reliable way. Keep in mind
that I've never used OCaml's module system.

Maybe that's just a lot harder to fit into GHC as it is today, though.
But I feel like the problem you're solving has already been solved by
first-class modules - type classes are for something else altogether.
But maybe we need a better example to argue about :-)

Antoine