State and Continuation

vczh edited this page Jan 30, 2014 · 3 revisions

Using state and continuation in Tinymoe is not a simple topic. So if you are interested in it, you should read:

Here we only introduce the syntax part of state and continuation.

If you have read Manipulating Functions, you may know that there is a special kind of functions called continuation. In order to touch the real state object and real continuation object, we introduce the cps for sentence and block.

  • For sentences, the syntax is cps (state) (continuation).
  • For blocks, the syntax is cps (state).

The declaration introduces two new variables. You can read and write fields of them. But changing these variables to reference to a new object does not work.

###State state is a variable storing extra information of the control flow event that is happening. The type of state is always:

type continuation state
	flag
	argument
	continuation
	trap
end

For the trap field, its type is always:

type continuation trap
	continuation
	previous trap
end

If the current control flow event supports pause and resume, then the type of the argument field should be:

type continuation fall back argument
	value
	trap
	fall back counter
end

Coroutine is one application of the pause and resume feature.

Details of trapping can be found in Standard Library: Trap.

Details of coroutine can be found in Standard Library: Coroutine.

When there is no happening control flow event, the flag, argument and continuation fields are all null. When there is, the flag field stores a value of symbol to specify the event name, and the values in argument and continuation depend on it.

If the continuation field is not null, it should be a continuation.

###Continuation Continuations are only accessable in sentences with cps (state) (continuation). If the continuation field of the state variable is not null, then the original value should comes from a cps sentence.

Continuation means "the next step". That means, if the following code is executing:

cps (state) (continuation)
sentence do something
	xxx
end

phrase main
	set a to 1
	do something
	set b to 2
	print a + b
end

When the running code reaches xxx, then calling the continuation means executing

set b to 2
print a + b

You are know understand what is a continuation. Continuation means "the next step", which means "everything that is going to be happened after the current cps sentence finished".

If you get the state and continuation, you can implement any control flow that make sense. To do that, you should read:

###Notice Dealing with continuation is not difficult, but it is complicated. You should follow all rules that are described in the two links before. Standard library functions are always your first choice. You don't want to invent strange things. If you really want to invent your own pattern of using continuation, make sure that your code is compatible with those standard library things, like try-catch and coroutine.

You are suggested to use standard library functions for state and continuation, if possible.

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