6.13.5.2 Shift, Reset, and All That

There is a whole zoo of delimited control operators, and as it does not seem to be a bounded set, Guile implements support for them in a separate module:

(use-modules (ice-9 control))

Firstly, we have a helpful abbreviation for the call-with-prompt operator.

Scheme Syntax: % expr

Scheme Syntax: % expr handler

Scheme Syntax: % tag expr handler

Evaluate expr in a prompt, optionally specifying a tag and a handler. If no tag is given, the default prompt tag is used.

If no handler is given, a default handler is installed. The default handler accepts a procedure of one argument, which will be called on the captured continuation, within a prompt.

Sometimes it’s easier just to show code, as in this case:

(define (default-prompt-handler k proc)
  (% (default-prompt-tag)
     (proc k)
     default-prompt-handler))

The % symbol is chosen because it looks like a prompt.

Likewise there is an abbreviation for abort-to-prompt, which assumes the default prompt tag:

Scheme Procedure: abort val1 val2 …

Abort to the default prompt tag, passing val1 val2 … to the handler.

As mentioned before, (ice-9 control) also provides other delimited control operators. This section is a bit technical, and first-time users of delimited continuations should probably come back to it after some practice with %.

Still here? So, when one implements a delimited control operator like call-with-prompt, one needs to make two decisions. Firstly, does the handler run within or outside the prompt? Having the handler run within the prompt allows an abort inside the handler to return to the same prompt handler, which is often useful. However it prevents tail calls from the handler, so it is less general.

Similarly, does invoking a captured continuation reinstate a prompt? Again we have the tradeoff of convenience versus proper tail calls.

These decisions are captured in the Felleisen F operator. If neither the continuations nor the handlers implicitly add a prompt, the operator is known as –F–. This is the case for Guile’s call-with-prompt and abort-to-prompt.

If both continuation and handler implicitly add prompts, then the operator is +F+. shift and reset are such operators.

Scheme Syntax: reset body1 body2 …

Establish a prompt, and evaluate body1 body2 … within that prompt.

The prompt handler is designed to work with shift, described below.

Scheme Syntax: shift cont body1 body2 …

Abort to the nearest reset, and evaluate body1 body2 … in a context in which the captured continuation is bound to cont.

As mentioned above, taken together, the body1 body2 … expressions and the invocations of cont implicitly establish a prompt.

Interested readers are invited to explore Oleg Kiselyov’s wonderful web site at http://okmij.org/ftp/, for more information on these operators.