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10.5 Continuation Marks

See Continuation Frames and Marks and Prompts, Delimited Continuations, and Barriers for general information about continuation marks.

The list of continuation marks for a key k and a continuation C that extends C0 is defined as follows:

• If C is an empty continuation, then the mark list is null.

• If C’s first frame contains a mark m for k, then the mark list for C is (cons m lst), where lst is the mark list for k in C0.

• If C’s first frame does not contain a mark keyed by k, then the mark list for C is the mark list for C0.

The with-continuation-mark form installs a mark on the first frame of the current continuation (see Continuation Marks: with-continuation-mark). Procedures such as current-continuation-marks allow inspection of marks.

Whenever Racket creates an exception record for a primitive exception, it fills the continuation-marks field with the value of (current-continuation-marks), thus providing a snapshot of the continuation marks at the time of the exception.

When a continuation procedure returned by call-with-current-continuation or call-with-composable-continuation is invoked, it restores the captured continuation, and also restores the marks in the continuation’s frames to the marks that were present when call-with-current-continuation or call-with-composable-continuation was invoked.

Returns an opaque value containing the set of continuation marks for all keys in the continuation

(or the current continuation of

if it is a thread) up to the prompt tagged by

. If

is

, the resulting set of continuation marks is empty. If

is an escape continuation (see

), then the current continuation must extend

, or the

exception is raised. If

was not captured with respect to

and does not include a prompt for

, the

exception is raised. If

is a dead thread, the result is an empty set of continuation marks.

Returns an opaque value containing the set of continuation marks for all keys in the current continuation up to prompt-tag. In other words, it produces the same value as

Returns a newly-created list containing the marks for

in

, which is a set of marks returned by

or

as a shorthand for

. The result list is truncated at the first point, if any, where continuation frames were originally separated by a prompt tagged with

. Producing the result takes time proportional to the size of the continuation reflected by

.

Changed in version 8.0.0.1 of package base: Changed to allow mark-set as #f.

Returns a newly-created list containing vectors of marks in

for the keys in

, up to

, where a

value for

is equivalent to

. The length of each vector in the result list is the same as the length of

, and a value in a particular vector position is the value for the corresponding key in

. Values for multiple keys appear in a single vector only when the marks are for the same continuation frame in

. The

argument is used for vector elements to indicate the lack of a value. Producing the result takes time proportional to the size of the continuation reflected by

times the length of

.

Changed in version 8.0.0.1 of package base: Changed to allow mark-set as #f.

Like

, but instead of returning a list of values, returns a functional iterator in the form of a procedure that returns one element of the would-be list and a new iterator function for the rest of the would-be list. An iterator procedure returns

instead of a vector when no more elements are available; in that case, the returned iterator procedure is like the called one, producing no further values. The time required for each step is proportional to the length of

times the size of the segment of the continuation reflected by

between frames that have keys in

.

Added in version 7.5.0.7 of package base.
Changed in version 8.0.0.1: Changed to allow mark-set as #f.

The result is produced in (amortized) constant time. Typically, this result can be computed more quickly using continuation-mark-set-first than using continuation-mark-set->list or by using continuation-mark-set->iterator and iterating just once.

Although #f and (current-continuation-marks prompt-tag) are equivalent for mark-set, providing #f as mark-set can enable shortcuts that make it even faster.

This function could be implemented with a combination of with-continuation-mark, current-continuation-marks, and continuation-mark-set->list*, as shown below, but call-with-immediate-continuation-mark is implemented more efficiently; it inspects only the first frame of the current continuation.

The optional sym argument, if provided, is used when printing the continuation mark.

Returns a list representing an approximate “

stack trace” for

’s continuation. The list contains pairs if

is

, where the

of each pair contains either

or a symbol for a procedure name, and the

of each pair contains either

or a

value for the procedure’s source location (see

); the

and

are never both

. If

is true, the list contains 3-element vectors, where the first two elements are like the values for a pair, and the third element is a realm symbol.

Conceptually, the stack-trace list is the result of continuation-mark-set->list with mark-set and Racket’s private key for procedure-call marks. The implementation may be different, however, and the results may merely approximate the correct answer. Thus, while the result may contain useful hints to humans about the context of an expression, it is not reliable enough for programmatic use.

A stack trace is extracted from an exception and displayed by the default error display handler (see error-display-handler) for exceptions other than exn:fail:user (see raise-user-error in Raising Exceptions).

Examples:

'(mark)

'((mark1) (mark2))

'(mark2)

'((mark2 mark1))

'(1)

Changed in version 8.4.0.2 of package base: Added the realms? argument.

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