[Haskell-cafe] Re: Pickling HList

[Keean Schupke]

This function is already in the HList library (well early versions 
anyway)... I dont think
this is in the current distribution. Its  a generic constructor  
wrapper. For example:

     hMarkAll Just hlist

    class HList l => HMarkAll c l m | c l -> m where
       hMarkAll :: (forall a . a -> c a) -> l -> m
    instance HMarkAll c HNil HNil where
       hMarkAll _ _ = HNil
    instance HMarkAll c l m => HMarkAll c (HCons e l) (HCons (c e) m) where
       hMarkAll c (HCons e l) = HCons (c e) (hMarkAll c l)

    Keean.

Joel Reymont wrote:

> Credit goes to Cale:
>
> class (HList l, HList p) => HLPU p l | p -> l, l -> p where
>     puHList :: p -> PU l
>
> instance HLPU HNil HNil where
>     puHList HNil = lift HNil
>
> instance (HList l, HLPU p l) => HLPU (HCons (PU e) p) (HCons e l) where
>     puHList (HCons pe l) =
>         wrap (\(a, b) -> HCons a b,
>               \(HCons a b) -> (a, b))
>                   (pair pe (puHList l))
>
>
> On Nov 10, 2005, at 2:04 PM, Joel Reymont wrote:
>
>> Folks,
>>
>> I'm having trouble creating a pickler for HLists and would  
>> appreciate a solution.
>>
>> The code for (HCons e HNil) works fine but I get an error trying to  
>> implement puHList for (HCons e l) where l is supposed to be (HCons  e 
>> ...), i.e. another HList.
>>
>> Bar.hs:21:37:
>> Couldn't match the rigid variable e' against PU e'
>> `e' is bound by the instance declaration at Bar.hs:17:0
>>
>> Expected type: HCons (PU e) l Inferred type: HCons e l
>> In the first argument of puHList', namely l'
>>
>> In the second argument of pair', namely (puHList l)'
>>
>> Failed, modules loaded: none.
>>
>
> -- 
> http://wagerlabs.com/
>
>
>
>
>
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