RFC: Adding a Hashable type class and HashMap/HashSet data types to HP

Johan Tibell johan.tibell at gmail.com
Fri Nov 26 18:14:55 EST 2010

On Fri, Nov 26, 2010 at 8:35 PM, Bryan O'Sullivan <bos at serpentine.com> wrote:
> I think so. The purpose of having the Hashable typeclass is so that the
> universe of typed values that we can hash is open.

I don't disagree that being able to hash everything is nice, but I
don't think it's crucial. My main interest in having a Hashable type
class is so we can have containers that can be keyed by hashable
things, for the types where this make sense (e.g. string like types
where comparison is expensive.) If all that a Hashable type class
would give me is the ability to store ByteStrings and Texts in a
HashMap, that alone would be enough motivation for having one in my

> I notice that the Hashable class doesn't have a way to supply a seed. If it
> did, we could possibly chain the result of one hash as the seed of the next,
> allowing us to construct a complete hash in an obvious way (although
> introducing a possibly undesirable data dependency, making it difficult or
> impossible to hash a large structure in parallel).

Having a hashWith method that takes a seed sounds reasonable. I guess
a hash algorithm that doesn't make use of a seed (are there any?)
could just throw away the seed in the implementation of hashWith.


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