[Haskell-cafe] Data polymophism
Phlex
Phlex at telenet.be
Sat Jun 9 05:41:53 EDT 2007
Thanks to both Christian and Tomasz.
This whole thing seems a bit too hard for the intended purpose.
After much tries and errors, I think i'll go for closures returning
DbIndex items.
Something like this :
uniqueIndex bkf datas =
DbIndex{dbiInsertIndex=insertIndex, {- more methods -}}
where index = UniqueIndex bkf datas
insertIndex id item = do newIndex <- uniqueIndexInsert item index
return $ uniqueIndex bkf $ uiItems newIndex
-- more functions
multiIndex bkf datas =
DbIndex{dbiInsertIndex=insertIndex, {- more methods -}}
where index = MultiIndex bkf datas
insertIndex id item = do newIndex <- multiIndexInsert id item index
return $ multiIndex bkf $ miItems newIndex
-- more functions
This keeps the definitions related to each index type close together,
and allows for extention by adding new index types later on. Actually I
don't even need the MultiIndex and UniqueIndex types anymore, making the
code even shorter.
Thanks for your time,
Sacha
Tomasz Zielonka wrote:
> On Fri, Jun 08, 2007 at 07:49:20PM +0200, Tomasz Zielonka wrote:
>
>> On Fri, Jun 08, 2007 at 05:23:23PM +0200, Phlex wrote:
>>
>>> But i don't seem to find a way to get out of this DbIndex type to
>>> actually work on the enclosed index.
>>>
>>> for instance, this doesn't work:
>>> liftDbIndex (DbIndex index) fun = DbIndex (fun index)
>>>
>> The compiler probably can't infer higher-ranker types, so you have to
>> write you type signature explicitly. Try:
>>
>> liftDbIndex :: Index_ i2 a2 k2 =>
>> (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> i2) -> DbIndex -> DbIndex
>>
>
> Now I think that this type signature will be too restrictive. Ideally, i2, a2 and
> k2 would be existentially quantified, like
>
> liftDbIndex :: (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> (exists. Index_ i2 a2 k2 => i2)) ->
> DbIndex -> DbIndex
>
> AFAIK such type isn't supported by any Haskell compiler, so we have to
> use the existential quantification from DbIndex:
>
> liftDbIndex :: (forall a1 k1 i1. Index_ i1 a1 k1 => i1 -> DbIndex) -> DbIndex -> DbIndex
> liftDbIndex f (DbIndex i) = f i
>
> Best regards
> Tomek
>
>
>
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