[racket] [redex] side-condition matching upon errors

>
>
> As for backwards compatibility issues, I don't know how much people rely
>> on side-condition taking something not-#f/#t... Probably too much :)
>> (also, there could be a `side-condition-soft' variant that is not strict
>> in the boolean-ness, or vice-versa)
>>
>>
> Well, if we're willing to force clients to change their code, then it
> isn't hard for them to write their own metafunctions that turn non-#f
> things into #ts.
>
>  But I think you're right that just changing this is a good idea and
> backwards compatibility be damned. I'll put it on my list.
>

There's at least one situation in which non-booleans in side conditions are
important: debugging. Much of my Redex debugging involves adding code along
the lines of "(side-condition (displayln (term <something>)))". Forcing
booleans there would make that line even more verbose, and when it's being
written a lot, that pain adds up.

I'd be in favor of something like side-condition-soft as described above.
Or alternatively, it might make sense to have a "debug" clause that acts
like printf, or even a "racket-expression" clause that evaluates the given
racket expression, but does not affect the result of a metafunction the way
side-condition does.
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