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.