Provides a shallow view of a SEXP R value as an algebraic datatype. This is useful to define functions over R values in Haskell with pattern matching. For example:

toPair :: SEXP a -> (SomeSEXP, SomeSEXP)
toPair (hexp -> List _ (Just car) (Just cdr)) = (SomeSEXP car, SomeSEXP cdr)
toPair (hexp -> Lang car (Just cdr)) = (SomeSEXP car, SomeSEXP cdr)
toPair s = error $ "Cannot extract pair from object of type " ++ typeOf s

(See SomeSEXP for why we need to use it here.)

The view is said to be shallow because it only unfolds the head of the R value into an algebraic datatype. In this way, functions producing views can be written non-recursively, hence inlined at all call sites and simplified away. When produced by a view function in a pattern match, allocation of the view can be compiled away and hence producing a view can be done at no runtime cost. In fact, pattern matching on a view in this way is more efficient than using the accessor functions defined in Foreign.R, because we avoid the overhead of calling one or more FFI functions entirely.

HExp is the view and hexp is the view function that projects SEXPs into HExp views.