Hi,
On 02/23/2011 04:40 PM, Kurt Stutsman wrote:
> [...]
> 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 -}
You might want to use a wrapper type: (instead of the Serializable
instance above)
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
newtype ByEnum e = ByEnum { unByEnum :: e }
deriving (Eq, Ord, Read, Show, Enum) -- just for convenience
instance Enum e => Serializable (ByEnum e) where
get = ByEnum . {- same code as above -}
put = {- same code as above -} . unByEnum
To see why this can't be done as you tried above, say that you have
another instance of Serialize for types that are an instance of both
Show an Read, serializing to/from a string using the 'show' and 'read'
functions.
Then consider a type which is an instance of all Show, Read, and Enum,
for example:
data Food = Meat | Vegetables deriving (Show, Read, Enum)
Which instance of Serializable should be used? The first one that was
declared? Rather not...
An instance like
"If (Enum t), then (Serializable t) via the Enum instance; else, if
(Show t, Read t), then (Serializable t) via the Show and Read instances;
otherwise not (Serializable t)"
would be perfect, but unfortunately Haskell doesn't have a way to
express this (yet?). Some steps[1] in this direction can however be
taken with the current state of the language.
-- Steffen
[1] http://haskell.org/haskellwiki/GHC/AdvancedOverlap
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