[Haskell-cafe] instance Binary UTCTime (Was: Oprhan instances)

[Antoine Latter]

On Thu, Nov 26, 2009 at 2:27 PM, Joachim Breitner
<mail at joachim-breitner.de> wrote:
> PS at -cafe:
> I still have doubts that this approach will scale as hackage grows. We’d
> get a lot more dependencies than we have now, and still would face this
> problem every now and then. Or do you really want packages that provide
> data types (like time) depend on all the libraries that do some kind of
> conversion (binary, html, xhtml, json, yaml, etc.)?
>

It's something I wonder about myself. Here's a thought experiment:

Lets say I want to provide an alternate or additional library of monad
transformer data types. To make these types maximally useful, they
should implement the typeclasses in the mtl package and in the
monads-tf package.

The only way to do this in a reasonable way is with multiple packages
and orphan instances:

mypackage
mypackage-classes-tf
mypackage-classes-fd

where the 'classes' packages do nothing but provide class implementations.

But then we're in a tight spot if someone doesn't notice that I have
the mypackage-classes-tf package released, provides their own
instances, and ships them in a library.

Am I missing something? And how can we extend the language to make
this better? Does anything short of class-instance explicit
import/export make this better?

Antoine