http:bad_request

/usr/lib/swi-prolog/library/http/http_exception.pl

library
- http

http:bad_request_error(+Formal, -ContextTemplate) is semidet[multifile, library(http/http_exception)]

If an exception of the term

error(Formal, context(Stack,
Context))

is caught and subsumes_term(ContextTemplate, Context) is true, translate the exception into an HTTP 400 exception. If the exception contains a stack-trace, this is stripped from the response.

The idea behind this hook is that applications can raise 400 responses by

Throwing a specific (error) exception and adding a rule to this predicate to interpret this as 400.
Define rules for error_message//1 to formulate an appropriate message.