Unordered map

Andrew Wagner wagner.andrew at
Tue Jul 29 16:20:14 EDT 2008

Ah, good call. I don't have an interpreter handy, but I'm sure you're
right. What about the other method of making the library more
consistent? One that assumes some default (non-)ordering?

On Tue, Jul 29, 2008 at 2:05 PM, Neil Mitchell <ndmitchell at> wrote:
> Hi
>> The other way to make the library more consistent, perhaps, would be
>> to simply move the Ord requirement up to that data structure: that is,
>> make it Ord k => Map k a, and then have a new data structure like
>> UnorderedMap (or, to use a more standard term, Dictionary).
> Have you tried to do this? You get errors all over the place, the
> monomorphism restriction kicks in, and it goes really wrong. Consider
> Data.Map.empty, you'd have to pass an Ord dictionary for the key,
> which often isn't known at that point. Suddenly the code:
> newMap = Data.Map.empty
> Stops working! Having too much polymorphism at random places breaks it.
> In essence, putting a context on a data type is a really bad idea.
> Haskell's solution with Data.Map is perfectly fine, and seems logical
> once you realise that its just the Haskell encoding of Ord k => Map k
> a
> Thanks
> Neil

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