Allow predicate error messages to be functions

[bhb]

The second arg to expound/def could either be a string or a function that is passed the problem (and maybe the spec itself?) and returns an error string. I believe that would allow you to grab any information you’ve added to the problem and would also allow you to include any relevant information (e.g. the range information) in the message.

https://clojureverse.org/t/improving-error-messages-in-clojure-as-a-library/1765/14?u=bbrinck

[bhb]

Use case: spec-tools allows users to add a "reason" to specs https://github.com/metosin/spec-tools/blob/master/README.md#custom-errors

[bhb]

Closing because I don't see a lot of demand for this feature.