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where <expr> is any valid KRL expression that results in a string (or something that can be cast as a string such as a number).

Explicit logging places the log message in two places:

• The log message is placed in the transaction log that is returned with the evaluation results if the logging pragma is on in the Meta Sectionmeta section. 
• The log message is shown in the browser's console if the ruleset returns JavaScript to a browser (see Web actions). 

The following example would place a string with the value of a variable named query in the log if the rule fired:

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The following rule is selected when an http:post() event is raised indicating seen with a error status code indicating an error. The error is logged and then processing is stopped. 

rule r3 is active {
  select when http post label re#ex# status_code re#([45]\d\d)# setting (status)
  noop();
  fired {
    log <<Error: #{status}: #{event:attr("status_line")}>>
    last;
  }
} 

Another approach to logging would use explicit error handling.

See Debugging KRL for more information on debugging tools.