Racket: The default error display handler does not recognize prop:exn:srclocs

[willghatch]

What version of Racket are you using?
7.9

What program did you run?

#lang racket/base
(require racket/match)
(match 'foo
  ['bar 'quux])

or any program where a match fails.

What should have happened?

If we run this with Racket BC, we get the following error info:

match: no matching clause for 'foo
  location...:
   match-error-example.rkt:3:0
  context...:

But in Racket CS, the location part is missing:

match: no matching clause for 'foo
  context...:

I guess I hadn't had match errors when using Racket CS before today (I only recently switched to defaulting to CS for most development), and I was surprised at the sudden lack of location information. I can imagine this was removed for Racket CS potentially as some kind of performance optimization, but I didn't see anything in the docs about it, or any way to re-enable it.

I'm frequently writing debug macros that give me more source information when an error is raised, and match was one macro that I never needed to do that with. Maybe this means I just don't know how to use debug tools like errortrace very well. But this error location from the match macro was super helpful.

[samth]

match definitely isn't doing anything different on CS, so this is likely just a general CS vs BC issue.

[willghatch]

match definitely isn't doing anything different on CS, so this is likely just a general CS vs BC issue.

I thought that was probably the case. I guess for some reason I felt like I should make some kind of argument just in case this was an intentional change.

[shhyou]

Can confirm the difference with this code:

#lang racket/base

(struct exn/loc exn:fail ()
  #:property prop:exn:srclocs
  (λ (e) (list (srcloc "source.rkt" 3 1 25 5))))

(raise (exn/loc "here" (current-continuation-marks)))

Racket v7.9 [bc]:

$ racket -v; racket srcl.rkt 
Welcome to Racket v7.9 [bc].
here
  location...:
   source.rkt:3:1
  context...:
   "/Volumes/ramdisk/srcl.rkt": [running body]

Racket CS:

$ racket -v; racket srcl.rkt 
Welcome to Racket v8.0.0.1 [cs].
here
  context...:
   /Volumes/ramdisk/srcl.rkt:7:0
   body of "/Volumes/ramdisk/srcl.rkt"

[sorawee]

I believe this is the same as #3325, but that issue is primarily focused on error message in DrRacket.

[shhyou]

It's not obvious if the two are related or not. DrRacket preprocesses source locations before highlighting the editor, and it obtains source locations directly from the prop:exn:srclocs property. So the source location information is still there, but is dropped by the preprocessing function.

https://github.com/racket/drracket/blob/6cda6a7b1738b0be624f96aba10c48b1e5adbe4d/drracket/drracket/private/rep.rkt#L637-L644
In (define/public (highlight-errors raw-locs [raw-error-arrows #f]) ...):

raw-locs = (#(struct:srcloc #<path:exexns.rkt> 42 17 1302 9))
locs after (cleanup-locs raw-locs) = ()

[shhyou]

Figured one difference for DrRacket:

In Racket CS, the source location is a relative path. In Racket v7.9 [bc], the source location is an absolute path:

However, this difference only appears when running from DrRacket.

(In Racket v7.9 [bc])
raw-locs = (#(struct:srcloc #<path:/Volumes/ramdisk/exexns.rkt> 42 17 1302 9))
locs after (cleanup-locs raw-locs) = (#(struct:srcloc #(struct:object:add-on-paint-logging ...) 42 17 1302 9)); 

[mflatt]

I'm working on the prop:exn:srclocs problem.