In certain cases, OmniMark creates coroutines in order to efficiently stream data within an OmniMark program. Most of the time, this behavior is handled completely automatically and there is no need for the programmer to worry about it. However, there are occasions in which the behavior of coroutines may lead to unexpected behavior in your program. This topic will help you understand when coroutines are active and what consequences they have for your program.
Coroutining occurs when you have one routine in you program that provides a stream of data and a second routine that consumes that data. Under normal circumstances, the first routine would run to completion, its output would be buffered, and when it was finished, the second routine would start and consume the data from the buffer. This is what happens, for instance, when you call a string returning function in OmniMark:
define string function get-some-text as local stream temp-buffer open temp-buffer as buffer using output as temp-buffer repeat for integer i from 1 to 1000 output "%d(i): abcdefghijklmnopqrstuvwxyz%n" again close temp-buffer return temp-buffer process submit get-some-text find lc+ => letters output "%ug(letters)"
In this program, the entire output of the function, the alphabet repeated one thousand times, is buffered in the
stream temp-buffer and returned to the
submit statement in a single block. However,
if you replace the string function with a
string source function you will get co-routining behavior:
define string source function make-some-text as repeat for integer i to 1000 output "%d(i): abcdefghijklmnopqrstuvwxyz%n" log-message "input" again process using output as file "out.txt" submit make-some-text find lc+ => l output "%ug(l)" log-message "main"
In this case, the
string source function runs as one coroutine and the calling function runs as another coroutine. Every time the output statement is executed in the
string source function, control is passed to the calling program where the submit statement processes the data returned and then passes control back to the
string source function. Control is passed back and forth between the two routines until the
string source function is completed. You can see this behavior very clearly by tracing the sample program in the debugger or by running it at the command line and observing how the log-message alternatively prints "input" and "main" on the console.
You can use a string source to generate or pre-process markup that is to be sent to a parser. When a
string source function is called in a parse context, the
string source function and the parser run as coroutines. That is, the
string source function is executed incrementally and streams its output to the parser incrementally. Execution passes back and forth between the parser and the
string source function until the entire input has been processed. This avoids buffering the entire output of the
string source function before the parser starts. This saves computer resources and improves performance. You can process very large amounts of data without running into resource problems.
In the following example, the
string source function parser-feeder generates an XML document by submitting a source. The markup is generated partly in the function itself and partly by find rules fired as a result of the
submit statement. The output of the function and the find rules becomes the input to the parser. Element rules then transform the XML into HTML.
define string source function parser-feeder as output "<greeting>" submit "Hello world." output "</greeting>" find "world" => planet-name output "<planet>" || planet-name || "</planet>" process do xml-parse scan parser-feeder output "<HTML><BODY>%c" || "</BODY></HTML>" done element "greeting" output "<P>%c</P>" element "planet" output "<B>%c</B>"
Other advantages of using
string source functions include:
In rare cases you may experience problems with the use of
string source functions or the
up-translate aided translation types because of the way OmniMark coordinates the activities of coroutines.
When you use coroutines, you have two processes running cooperatively within a single program. OmniMark runs each process in a separate processing domain. Some resources are owned by one domain or the other, while others are shared between the two domains.
If you experience an error in a program that uses coroutines, the answer may be found in the following:
next group isin one domain affects the active groups in the other domain. A scope protected with
using group, however, isolates the active group from changes done to active groups in other domains. Changing groups in the isolated scope does not affect the active groups in any other domain, nor vice versa. Parsing constructs like
do sgml-parse, as well as the aided translation types, save groups in their input domain automatically. Each domain of a parsing construct has its own active groups.
globalvariable, the new value will be immediately visible by all other domains, even if the variable is saved by the modifying domain. For this reason,
save-clearoperations must nest properly when they're applied to a plain
globalvariable. Because domains are parallel, and not nested within each other, a save in one domain may not be properly nested with respect to a save of the same variable in the other domain. This causes a run-time error with
globalvariables. If the variable modifications don't need to be visible to other domains, declare it as
domain-bound global. The
save-clearactions can be applied to a
domain-bound globalvariable without the nesting restriction.
using nested-referentsin one domain at a time. In no circumstance can you output referents to the parser itself.
Note that it may not always be obvious which coroutine a certain piece of code is running in. For instance, a
find rule could be fired either by a
submit in an
string source function or by a
submit in an element rule. That rule would be running in one domain in the first case and in the other domain in the second case.
If you write code that depends on the timing of the switching between the
string source function and the parser, you may need to be aware of the rules OmniMark uses when switching domains.
string sourcefunction, you can test to determine what the current element is in the domain of the parser using the
string sourcefunction at the end of every output statement that outputs to the
In versions of OmniMark prior to version 7, an input function was an action function which was called as an input function by adding the keyword
input to the parser invocation:
define function parser-feeder as output "<greeting>" submit "Hello world." output "</greeting>" find "world" => planet-name output "<planet>" || planet-name || "</planet>" process do xml-parse scan input parser-feeder output "<HTML><BODY>%c" || "</BODY></HTML>" done element "greeting" output "<P>%c</P>" element "planet" output "<B>%c</B>"
This form is deprecated but is supported for backward compatibility.