[Haskell-cafe] Recommended class instances for container type

[wren ng thornton]

On 12/8/11 11:12 AM, Christoph Breitkopf wrote:
> Hello,
>
> 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,
> ...).
>
> And what about the more experimental things? Say, DeepSeq, Typeable, Data?
> I'd like to keep this simple at start, and I've admittedly not followed
> recent developments in Haskell-land (recent meaning the last 10 years or
> so. I _do_ know about hierachical modules ;-) ).

I don't use Typeable or Data, but there are a lot of folks who do, and 
they seem pretty well entrenched in GHC-standard Haskell. Not sure about 
non-GHC compiler support.

For a container datatype, I'd consider Foldable and Traversable to be 
essential (provided that they're actually implementable). These classes 
are widely used and so they offer a nice standard set of names for 
common operations. But more than just having a common set of names, 
implementing these classes ensures a minimum level of completeness for 
your API--- and that's the essential part.

Functor, Applicative, and Monad are also good to offer whenever 
possible. Functor is required/implied by Foldable and Traversable. 
Applicative and Monad just give a nice clean interface--- though you 
should beware of whether there are multiple law-abiding implementations 
for these two classes. If there are, then you'll have to worry about 
which one you offer by default (if any) as well as how people can access 
the other ones.

-- 
Live well,
~wren