printing errors

[Shriram Krishnamurthi]

Given an exception, is there a way to print the error using Racket's conventional error printing machinery (e.g., in color in DrRacket, etc.), without halting execution?

I would like to be able to integrate this with #%printing-module-begin and #%top-interaction. Unfortunately, those by default will halt execution after printing the message.

[Robby Findler]

Yes, something like this:

(define (print-exn exn)
    ((error-display-handler)
     (if (exn? exn)
         (exn-message exn)
         (format "~a" exn))
     exn))

On Thu, Aug 27, 2020 at 3:31 PM Shriram Krishnamurthi <shr...@gmail.com> wrote:

Given an exception, is there a way to print the error using Racket's conventional error printing machinery (e.g., in color in DrRacket, etc.), without halting execution?

I would like to be able to integrate this with #%printing-module-begin and #%top-interaction. Unfortunately, those by default will halt execution after printing the message.

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[Shriram Krishnamurthi]

This is perfect, thanks!

Two follow-up questions:

1. The error printer seems to print an extra newline at the end relative to what the port-display-handler (for instance) shows. Is there a way to suppress that?

2. The stack trace seems to be extracted automatically. Is there a way to suppress it entirely?

Shriram 

[Sorawee Porncharoenwase]

For stacktrace, use error-print-context-length to suppress it:

(define (print-exn exn)
  (parameterize ([error-print-context-length 0])
    ((error-display-handler)
     (if (exn? exn)
         (exn-message exn)
         (format "~a" exn))
     exn)))

I don’t see any extra newline though.

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[Robby Findler]

It may be that you're seeing the "clever"ness of the DrRacket repl. If the last character printed is a newline, it only shows it when there is another character after the newline. 

Robby

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[Shriram Krishnamurthi]

Thank you!

Is there a way of further suppressing info? Right now I get output like

 ../../make-semantics.rkt:37:13: a: undefined

which is a reference to the language implementation file rather than to the program in the language.

The programs here are so small that suppressing everything but the "a: undefined" would be great.

[Robby Findler]

It might make the most sense for you to extract the exn-message and print it yourself? It should contain only the "a: undefined" part. If you want to selectively access the stacktrace and decide whether or not to print that first stackframe, that'll probably require a change to DrRacket, tho -- at least if you have debugging turned on. (There may be some clever hack I'm not thinking of, tho, depending on what you want.)

Robby

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[Sorawee Porncharoenwase]

Alternatively, I think you can simply supply a non-exception value to the handler. This will suppress the red cross icon too, though.

(define (print-exn exn)
  ((error-display-handler)
   (if (exn? exn)
       (exn-message exn)
       (format "~a" exn))
   #f))