The culprit is that 'err is defined to be Racket's 'error. It looks like every single use case of 'error is discouraged for one reason or another in the Racket reference:
- (error sym) creates a message string by concatenating "error: " with the string form of sym. Use this form sparingly.
- (error msg v ...) creates a message string by concatenating msg with string versions of the vs (as produced by the current error value conversion handler; see error-value->string-handler). A space is inserted before each v. Use this form sparingly, because it does not conform well to Racket’s error message conventions; consider raise-arguments-error, instead.
- (error src frmat v ...) creates a message string equivalent to the string created by
(format (string-append "~s: " frmat) src v ...)
When possible, use functions such as raise-argument-error, instead, which construct messages that follow Racket’s error message conventions.
Er, I knew it was weird for me to say "'err is defined to be Racket's 'error," but I just realized, that factoid was in the original post of this thread. :-p
