Your Map' (==) is lying! :)
Your definition purports to establish an equivalence class for all MP
(key,value) with the same key, but MP(key,1) and MP(key,2) are not
"equivalent" in any meaningful way outside the internals of Map' (else
you could dispense with the payload entirely!)
Set is now not a representation of Map', but a co-representation.
Details are exposed to outsiders to hide them from Map'. Everyone else
pays so that Map' 's life is a little easier.
Contrast that with, say, a set represented by a list, with compare
defined to sort before comparing. This is a meaningful (to outsiders)
equivalence relation because it hides the internal representation
artifact that lists have a (spurious) ordering.
IMHO the interface should represent the external properties, not some
internal invariant. In short, Map' doesn't say what it mean and mean
what it says. If you told me for a, b :: MyPair k v that a == b, I would
(foolishly) expect that a = b. I suspect that I wouldn't be the only one
to make that mistake.
Dan Weston
Tony Morris wrote:
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...it seems to be a special case of Set? Does Data.Map add anything more
useful than Map' below?
import Data.Set as Set
newtype MyPair a b = MP (a, b)
deriving Show
instance (Eq a) => Eq (MyPair a b) where
MP (a, _) == MP (a', _) = a == a'
instance (Ord a) => Ord (MyPair a b) where
MP (a, _) `compare` MP(a', _) = a `compare` a'
type Map' k a = Set (MyPair k a)
- --
Tony Morris
http://tmorris.net/
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