Daniel Fischer wrote:
No, it's not. The language report says an instance head must have the form
(tyCon a1 ... an),
where tyCon is a type constructor and a1 ... an are *distinct* type
variables (appropriate in number so that the head has the correct kind).
In instance (Enum e) => Test e where ..., the tyCon is not present.
Since this is too restrictive for many cases, most implementations have
extensions allowing more liberal instance declarations (omitting the tyCon
part, allowing repeated type variables, ...).
Note however, that the above instance means "all types are instances of
Test, and using a Test method on a type which doesn't belong to Enum is a
static error" in GHC [because the instance selection in GHC doesn't take
the part before the '=>' into account, so it sees 'instance Test e where'].
If you want to declare any other instances of Test, you need to enable
OverlappingInstances, which is a whole 'nother can of worms.
Excellent! That was just the kind of information I was looking for. Thanks.
Going back to my original problem then, I am encoding and decoding from
a file that contains many bitsets. In my Haskell code, I am using
Data.BitSet in conjunction with Enums I am creating for each kind of
bitset. I thought the syntax I was using before would be perfect for
using the same code to transcode between the bitmask integer and the
internal representation. Test is actually a kind of Serializable class.
I don't want to restrict it to only working with Enums, which is what
your OverlappingInstances seems to address. Is there a better way for
doing what I am trying to do?
Example:
import Data.BitSet
data GroupA = A1 | A2 | A3 deriving (Enum, Show)
data GroupB = B1 | B2 deriving (Enum, Show)
class Serializable t where
get :: String -> t
put :: t -> String
instance Enum e => Serializable e where
get mask = {- convert mask to Int and then to a BitSet -}
put bitset = {- convert BitSet to Int and then to String -}
Thanks,
Kurt
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