Unordered map

[Johannes Waldmann]

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> [...] Two different languages, two different
> approaches to implementing collections---one more imperative, the other
> more functional.

It's "easier" to implement Hash than Ord,
because for Ord, you need transitivity.
while a hash function just needs to be a function.
(Difficult in Java, impossible to get wrong in Haskell.)
If you take a bad hash function,
you're "only" hurting performance, not correctness.

In fact I sometimes make a wrapper type sth. like
Wrap { hash = hash x, contents = x } deriving ( Eq, Ord )
(hoping for the left-to-right comparison)
and then use Data.Map.

Best regards, J.W.
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