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.
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