Re: Question about sets

Sets of arbitrary elements are not possible, because, for instance, you need to be able to compare elements to ensure no duplicates in the set and you can't do this for any arbitrary pair of types. Lists with arbitrary elements are possible, but not very useful. After all, what could you do with them? You don't know what type they are so you couldn't apply any function to them, except for a function which doesn't depend in any way on the type of its argument and hence doesn't depend on its argument at all. Any example of such lists would be: data AnyList = AnyNil | forall a . AnyCons a AnyList But I don't think this is what you really want. If you need to be able to store any two types in a list/set, use a union datatype, like: data Either a b = Left a | Right b then you can have a set of, for example, Either Int String. You can generalize this on your own. On Tue, 20 Aug 2002, Scott J. wrote:

As far a I know sets can implemented by implementing a list of anything(a list of all types) The sets Haskell does have are AFAIK sets of elements of the same type: these are not general sets.

Scott

----- Original Message ----- From: "Ketil Z. Malde" To: Sent: Tuesday, August 20, 2002 8:56 AM Subject: Re: Question about sets

...

"Scott J." writes:

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I have a question. Why are sets not implemented in Haskell?

What do you mean? Isn't

http://www.haskell.org/ghc/docs/latest/html/hslibs/set.html

sufficient? (Remember to tell GHC '-package data')

-kzm -- If I haven't seen further, it is by standing in the footprints of giants _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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