/usr/lib/swipl/library/http/http_header.pl
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 http:status_page(+Status, +Context, -HTMLTokens) is semidet[multifile, library(http/http_header)]
Hook called by http_status_reply/4 and http_status_reply/5 that allows for emitting custom error pages for the following HTTP page types:
  • 201 - created(Location)
  • 301 - moved(To)
  • 302 - moved_temporary(To)
  • 303 - see_other(To)
  • 400 - bad_request(ErrorTerm)
  • 401 - authorise(AuthMethod)
  • 403 - forbidden(URL)
  • 404 - not_found(URL)
  • 405 - method_not_allowed(Method,URL)
  • 406 - not_acceptable(Why)
  • 500 - server_error(ErrorTerm)
  • 503 - unavailable(Why)

The hook is tried twice, first using the status term, e.g., not_found(URL) and than with the code, e.g. 404. The second call is deprecated and only exists for compatibility.

Arguments:
Context- is the 4th argument of http_status_reply/5, which is invoked after raising an exception of the format http_reply(Status, HeaderExtra, Context). The default context is [] (the empty list).
HTMLTokens- is a list of tokens as produced by html//1. It is passed to print_html/2.