debug messages
The SWI-Prolog library(debug) provides debug/3 for printing debug
messages. The signature is debug(+Channel, +Format, +Args), where
Format and Args are passed to format/2. The Channel may be
used to enable or disable the message and determine where it is
printed to. This is controlled using debug/1 and nodebug/1.
The debug messages GUI can be launched from the Epilog
Tools/Debug messages menu. It has two panes. The left pane shows
the known debug channels and the right pane collects the debug
messages. Clicking a channel in the left pane highlights all messages
related to this channel. The context menu allows enabling or disabling
channels, i.e., providing a GUI binding to debug/1 and nodebug/1.
Note Calls to debug/3 are removed if the code is compiled with
optimization enabled. By default, all SWI-Prolog libraries are
precompiled with optimization enabled. To use debug channels used by
the libraries, you should start Prolog using the -Dsource option or use
?- set_prolog_flag(source, true).
before loading the library you want to debug. This notably applies for the HTTP client and server libraries. These libraries provide several debug channels that may be used to debug connection issues.