[Haskell-cafe] Recommended class instances for container type

[Bryan O'Sullivan]

On Thu, Dec 8, 2011 at 8:12 AM, Christoph Breitkopf <
chbreitkopf at googlemail.com> wrote:

>
> I'm in the process of implementing a container data type, and wonder what
> class instances are generally considered necessary. E.g. is it ok to start
> out with a Show that's adequate for debugging, or is it a 'must' to include
> instances of everything possible (Eq, Ord if possible, Read, Show, Functor,
> ...).
>

If you're only beginning or partway through the implementation, my advice
would be to simply not worry about instances at all just yet, until you've
got things in reasonable shape. When the time comes, implement the
instances you think appropriate in order of importance, relevance, and
difficulty.

Of course if you're new to the community, it won't be too obvious what's
important or relevant (difficulty should be obvious enough). Basically, aim
at the standard classes first, based on how often you'd expect to use them
yourself.

And what about the more experimental things? Say, DeepSeq, Typeable, Data?
>

None of those are experimental. They're all frequently used in production
code. DeepSeq is far more important than the other two, though. For
Typeable and Data, you could copy the approach taken by Data.Map and be
fine.

At some point, if you want your container class to be useful to others,
you'll want to implement Foldable and Traversable.
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