Adrian Hey wrote:
[EMAIL PROTECTED] wrote:
Adrian Hey wrote:
-- Instances of GT are instances of Eq --
instance (GT map key, Eq a) => Eq (map a) where
map1 == map2 = assocsAscending map1 == assocsAscending map2
...
Overlapping instances for Eq [(key, a)]
arising from use of `==' at Test.hs:10:16-59
Matching instances:
instance (Eq a) => Eq [a] -- Defined in GHC.Base
instance (GT map key, Eq a) => Eq (map a) -- Defined at
Test.hs:9:0
How can my new instance overlap with the old (ghc) instance unless
[] is also an instance of GT for some key type (which it isn't).
Could someone explain?
You are right in your explanation of the GHC behavior. Your instance
|Eq (map a)| indeed overlaps the `standard` instance |Eq [a]|. The
overlap may be easier to see if we write [a] as ([] a), which is what
it is, an application of the type constructor [] to the type variable
a. So, the type [a] (aka [] a) is a particular instance of the type
(map a), with `map' being []. This is the overlapping that GHC is
complaining about.
So if I understand correctly, it's complaining about a potential
overlap, not an actual overlap (there is no actual overlap until []
is made an instance of GT AFAICS).
Also, I suspect I'm still missing something important here, for
example I don't understand why, if it overlaps for [], it doesn't
overlap with other instances (like Maybe for example). Or am I
just not getting the error for Maybe because ghc stops after
the first error?
Ah, this brings me to something else I don't quite understand
about this error, but kind of explains the absence of similar
errors for Maybe.
It's the "arising from use of `==' at Test.hs:10:16-59" part.
I don't understand why, but if I use..
-- Instances of GT are instances of Eq --
instance (GT map key, Eq a) => Eq (map a) where
map1 == map2 = True
..the error goes away. So it's not objecting to such instances
in general, but rather the specific use of == in the definition.
So now I'm really confused. Why, if it's possible that this
instance overlaps with another for [], does it make any
difference whether or not I've used == in the definition?
Surely, if there's any ambiguity about which instance of Eq to use
for [] (should [] be made an instance of GT), then the ambiguity
exists whether or not I've used == in the definition. Hmm..
Regards
--
Adrian Hey
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