### Controlling

[
  Datatype continuations
  Funcon   continuation
  Entity   plug-signal
  Funcon   hole
  Funcon   resume-continuation
  Entity   control-signal
  Funcon   control
  Funcon   delimit-current-continuation  Alias delimit-cc
]

Meta-variables
  T, T1, T2 <: values

Datatype
  continuations(T1,T2) ::= continuation(_:abstractions(()=>T2))
/*
  `continuations(T1, T2)` consists of abstractions whose bodies contain a `hole`,
  and which will normally compute a value of type `T2` when the `hole` is plugged
  with a value of type `T1`.
*/

Entity
  _ --plug-signal(V?:values?)-> _
/*
   A plug-signal contains the value to be filled into a `hole` in a continuation,
   thereby allowing a continuation to resume.
*/

 Funcon
  hole : =>values
/*
  A `hole` in a term cannot proceed until it receives a plug-signal
  containing a value to plug the hole.
*/
Rule
 hole --plug-signal(V)-> V

Funcon
  resume-continuation(K:continuations(T1, T2), V:T1) : =>T2
/*
 `resume-continuation(K, V)` resumes a continuation `K` by plugging the value
 `V` into the `hole` in the continuation.
*/
Rule
                  X --plug-signal(V)-> X'
 ---------------------------------------------------------------------------
 resume-continuation(continuation(abstraction(X)), V:T) --plug-signal()-> X'


Entity
  _ --control-signal(F?:(functions(continuations(T1, T2), T2))?)-> _
/*
   A control-signal contains the function to which control is about to be passed
   by the enclosing `delimit-current-continuation(X)`.
*/

Funcon
  control(F:functions(continuations(T1, T2), T2)) : =>T1
/*
  `control(F)` emits a control-signal that, when handled by an enclosing
  `delimit-current-continuation(X)`, will apply `F` to the current continuation of
  `control(F)`, (rather than proceeding with that current continuation).
*/
Rule
  control(F:functions(_,_)) --control-signal(F)-> hole


Funcon
  delimit-current-continuation(X:=>T) : =>T
Alias
  delimit-cc = delimit-current-continuation
/*
  `delimit-current-continuation(X)` delimits the scope of captured continuations.
*/
Rule
  delimit-current-continuation(V:T) ~> V
Rule
                         X --control-signal( )-> X'
  -----------------------------------------------------
  delimit-current-continuation(X) --control-signal( )->
    delimit-current-continuation(X')
Rule
                         X --control-signal(F)-> X'
  ------------------------------------------------------------------
  delimit-current-continuation(X) --control-signal( )->
    delimit-current-continuation(apply(F, continuation closure(X')))