Proposal: Allow gunfold for Data.Map, Data.IntMap, etc.

[José Pedro Magalhães]

+1


Pedro

On Wed, Aug 29, 2012 at 6:34 PM, Edward Kmett <ekmett at gmail.com> wrote:

> I would like to propose improving the Data instances for a number of
> currently completely opaque data types in the containers package, by using
> virtual constructors.
>
> The instance for Data.Map already uses fromList for gfoldl, it just stops
> there.
>
> Extending it to be able to gunfold and mention the name of that
> constructor would enable generic traversal libraries like uniplate, etc. to
> work over the contents of the Map, rather than bailing out in fear or
> crashing at the sight of a mkNoRepType.
>
> An example of the changes for Data.Map are highlighted below.
>
> instance (Data k, Data a, Ord k) => Data (Map k a) where
>   gfoldl f z m   = z fromList `f` toList m
>   toConstr _     = fromListConstr
>   gunfold k z c  = case constrIndex c of
>     1 -> k (z fromList)
>     _ -> error "gunfold"
>   dataTypeOf _   = mapDataType
>   dataCast2 f    = gcast2 f
>
> fromListConstr :: Constr
> fromListConstr = mkConstr mapDataType "fromList" [] Prefix
>
> mapDataType :: DataType
> mapDataType = mkDataType "Data.Map.Map" [fromListConstr]
>
> I've used this approach for years on my own libraries to great effect.
>
> Discussion Period: 3 Weeks
>
> (I added a week for ICFP)
>
> -Edward
>
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>
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