## 2.12 An overview of hook predicates

SWI-Prolog provides a large number of hooks, mainly to control handling messages, debugging, startup, shut-down, macro-expansion, etc. Below is a summary of all defined hooks with an indication of their portability.

- portray/1
  Hook into write_term/3 to alter the way terms are printed (ISO).
- message_hook/3
  Hook into print_message/2 to alter the way system messages are printed (Quintus/SICStus).
- message_property/2
  Hook into print_message/2 that defines prefix, output stream, color, etc.
- message_prefix_hook/2
  Hook into print_message/2 to add additional prefixes to the message such as the time and thread.
- library_directory/1
  Hook into absolute_file_name/3 to define new library directories (most Prolog systems).
- file_search_path/2
  Hook into absolute_file_name/3 to define new search paths (Quintus/SICStus).
- term_expansion/2
  Hook into load_files/2 to modify read terms before they are compiled (macro-processing) (most Prolog systems).
- goal_expansion/2
  Same as term_expansion/2 for individual goals (SICStus).
- prolog_load_file/2
  Hook into load_files/2 to load other data formats for Prolog sources from `non-file' resources. The load_files/2 predicate is the ancestor of consult/1, use_module/1, etc.
- prolog_edit:locate/3
  Hook into edit/1 to locate objects (SWI).
- prolog_edit:edit_source/1
  Hook into edit/1 to call an internal editor (SWI).
- prolog_edit:edit_command/2
  Hook into edit/1 to define the external editor to use (SWI).
- prolog_list_goal/1
  Hook into the tracer to list the code associated to a particular goal (SWI).
- prolog_trace_interception/4
  Hook into the tracer to handle trace events (SWI).
- prolog:debug_control_hook/1
  Hook in spy/1, nospy/1, nospyall/0 and debugging/0 to extend these control predicates to higher-level libraries.
- prolog:help_hook/1
  Hook in help/0, help/1 and apropos/1 to extend the help system.
- resource/3
  Define a new resource (not really a hook, but similar) (SWI).
- exception/3
  Old attempt to a generic hook mechanism. Handles undefined predicates (SWI).
- attr_unify_hook/2
  Unification hook for attributed variables. Can be defined in any module. See section 8.1 for details.