hlist-tuple provides a tuple-based interface to HList

creating an HList:

(e1) .*. (e2) .*. (e3) .*. HNil 

becomes

untuple (e1,e2,e3)

pattern matching on an HList:

let (HCons e1 (HCons e2 (HCons e3 HNil))) = l 

becomes

let (e1,e2,e3) = tuple l

---

HList-based libraries can add support for the tuple-based interface using the IsTuple type level predicate:

class SomeClass a b | a -> b where
  someF :: a -> b
instance SomeClass HNil ...
instance (SomeClass rest ...) => SomeClass (HCons e rest) ... 

becomes

instance (IsTuple l tup
         ,SomeClass' tup l ...           
         ) => SomeClass l ... where
  someF = someF' (undefined::tup)

instance (Untuple t l
         ,SomeClass l ...
         ) => SomeClass' HTrue t ... where 
  someF' _ t = someF $ untuple t

instance SomeClass' HFalse HNil ...
instance (SomeClass' HFalse rest ...
         ) => SomeClass' HFalse (HCons e rest) ... where
  someF' hFalse (HCons e rest) = ...

Where the (SomeClass' HFalse ...) instances are analagous to the original (SomeClass ...) ones.