A class for "liftable" data structures. Conceptually
an extension of
MonadTrans to allow deep lifting,
but lifting need not be done between monads only. Eg lifting
Applicatives is allowed.
For instances where
n have 'return'/'pure' defined,
these instances must satisfy
lift (return x) == return x.
This form of
lift has an extremely general type and is used primarily to
sample. Its excessive generality is the main reason it's not
exported from Data.Random.
RVarT is, however, an instance of
MonadTrans, which in most cases is the preferred way
to do the lifting.