Proposal: Add the unordered-containers package and the hashable package to the Haskell Platform

[Johan Tibell]

On Thu, Mar 21, 2013 at 2:26 PM, Isaac Dupree <
ml at isaac.cedarswampstudios.org> wrote:

> Regarding hashWithSalt determinism:
>
> hashable 1.1:
> "The general contract of hash is: * This integer need not remain
> consistent from one execution of an application to another execution of the
> same application. [...] The contract for hashWithSalt is the same as for
> hash, with the additional requirement that any instance that defines
> hashWithSalt must make use of the salt in its implementation." [1]
>
> hashable 1.2:
> "The general contract of hashWithSalt is: * If a value is hashed using the
> same salt during distinct runs of an application, the result must remain
> the same. (This is necessary to make it possible to store hashes on
> persistent media.) [...]"
>
> Which contract do we want?
>

When I wrote the 1.1 contract I gave me lots of leeway to change the
implementation in the future. The actual implementation was that the hash
was always the same between run to run for a given architecture.

I'm not terribly happy with the "This is necessary to make it possible to
store hashes on persistent media." part of the 1.2 contract. I should
probably not have let that go in. If you're persisting your hashes you
should use a hash function that guarantees exactly which algorithm is used.

I think the contract should be: the hash function is guaranteed to return
the same hash code for a given value as long as the code is compiled with
the same version of hashable, unless the user explicit turns on hash
randomization (i.e. random seed read from /dev/urandom). I don't think we
should make any guarantees that a new version of hashable won't change the
hash function used.

As for word sizes the only practical thing is to use the native word size,
as anything else is much too slow (i.e. Int64 is terribly slow on 32-bit
platforms).

-- Johan
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