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By convention, error messages (and in general, the reports generated by
write-condition-report) should consist of one or more complete
sentences. The usual rules for sentences should be followed: the first
word of the sentence should be capitalized, and the sentence should be
terminated by a period. The message should not contain extraneous
whitespace such as line breaks or indentation.
The error system provides a simple formatting language that allows the programmer to have some control over the printing of error messages. This formatting language will probably be redesigned in a future release.
Error messages typically consist of a string describing the error,
followed by some irritant objects. The string is printed using
display, and the irritants are printed using write,
typically with a space between each irritant. To allow simple
formatting, we introduce a noise object, printed using
display. The irritant list may contain ordinary objects
interspersed with noise objects. Each noise object is printed using
display, with no extra whitespace, while each normal object is
printed using write, prefixed by a single space character.
Here is an example:
(define (error-within-procedure message irritant procedure)
(error message
irritant
(error-irritant/noise "within procedure")
procedure
(error-irritant/noise ".")))
This would format as follows:
(error-within-procedure "Bad widget" 'widget-32 'invert-widget) error-->
Bad widget widget-32 within procedure invert-widget.
Here are the operations supporting error messages:
Message is typically a string (although this is not required), irritants a list of irritant objects, and port an output port. Formats message and irritants to port in the standard way. Note that, during the formatting process, the depth and breadth to which lists are printed are each limited to small numbers, to guarantee that the output from each irritant is not arbitrarily large.