Data.Fixed.Fixed constructor

[David Menendez]

On Thu, Jul 23, 2009 at 3:29 PM, Ashley Yakeley<ashley at semantic.org> wrote:
> On Thu, 2009-07-23 at 14:07 +0100, Jules Bean wrote:
>> Ashley Yakeley wrote:
>> > I'm currently working on updating the Data.Fixed module in base. I am
>> > adding Typeable and Data instances (automatically derived), and several
>> > new HasResolution types (including one for 10^-2, monetary currencies
>> > being an obvious Fixed use).
>>
>> Is an automatic Data instance wise?
>>
>> Morally speaking does a Data.Fixed really "include" a Integer?
>>
>> Compare the question: should the automatic Data instance for Complex a
>> be used, since this exposes the fact that Complex is stored using two
>> components in real/imaginary components?
>>
>> I think that abstract types like Complex and Fixed should be treated as
>> atoms by Data rather than being traversable, but I'm not sure.
>
> Could you give me an example of a Data instance for either Complex or
> Fixed that you approve of?
>
> Data instances unavoidably expose concrete structure. That may be the
> actual internal structure of the type, or it may be some constructed
> concrete structure, but it must be something.

For Fixed, perhaps something along the lines of the Double instance?

doubleType :: DataType
doubleType = mkFloatType "Prelude.Double"

instance Data Double where
  toConstr = mkFloatConstr floatType   -- that should probably be "doubleType"
  gunfold _ z c = case constrRep c of
                    (FloatConstr x) -> z x
                    _ -> error "gunfold"
  dataTypeOf _ = doubleType

You'd just need to insert some appropriate conversions between Double and Fixed.

Alternatively, you could make Fixed non-representable like Ptr,

instance Typeable a => Data (Ptr a) where
  toConstr _   = error "toConstr"
  gunfold _ _  = error "gunfold"
  dataTypeOf _ = mkNorepType "GHC.Ptr.Ptr"

Note that both these instances use the default definition for gfoldl,
which does not attempt to descend into the structure of the data. I
think this is appropriate, although whoever wrote the instance for
Ratio appears to disagree with me.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>