Monad. ST. Internal
ST is concerned with restricted mutation. Mutation is restricted to a
region of mutable references. This kind is inhabited by phantom types
which represent regions in the type system.
data ST :: Region -> Type -> Type
data ST t0 t1
ST type constructor allows local mutation, i.e. mutation which
does not "escape" into the surrounding computation.
ST computation is parameterized by a phantom type which is used to
restrict the set of reference cells it is allowed to access.
run function can be used to run a computation in the
run :: forall a. (forall r. ST r a) -> a
Note: the type of
run uses a rank-2 type to constrain the phantom
r, such that the computation must not leak any mutable references
to the surrounding computation. It may cause problems to apply this
function using the
$ operator. The recommended approach is to use