[Haskell-cafe] A curios monad
andrewcoppin at btinternet.com
Thu Dec 11 16:26:54 EST 2008
Alberto G. Corona wrote:
> but because keys and data must be of the same type, you are restricted
> to instances of Eq and Ord. Itsn´n true?
> 2008/12/11 Andrew Coppin <andrewcoppin at btinternet.com
> <mailto:andrewcoppin at btinternet.com>>
> Alberto G. Corona wrote:
> Well, you have not to use typeable, but instead you need to
> use instances of Eq and Ord. This is because the values and
> the keys must be of the same type. this is the tradeoff.
> Not really, no. The *keys* need Eq and Ord, but the values I'm
> storing don't. The values I'm storing don't need any specific
> properties at all. That's what I like about it.
No. Keys are of type "Key v", where "v" can be anything. It's a phantom
type, so it's not used for anything (except knowing what to coerce the
value back to when it's looked up). "Key v" is just an alias to Int,
which *is* in Eq and Ord, as required. There are no constraints on "v"
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