4.17.5 Adding IRI schemas

The file handling predicates may be hooked to deal with IRIs. An IRI starts with <scheme>://, where <scheme> is a non-empty sequence of lowercase ASCII letters. After detecting the scheme the file manipulation predicates call a hook that is registered using register_iri_scheme/3.

Hooking the file operations using extensible IRI schemas allows us to place any resource that is accessed through Prolog I/O predicates on arbitrary devices such as web servers or the ZIP archive used to store program resources (see section 14.2). This is typically combined with file_search_path/2 declarations to switch between accessing a set of resources from local files, from the program resource database, from a web-server, etc.

register_iri_scheme(+Scheme, :Hook, +Options)

Register Hook to be called by all file handling predicates if a name that starts with Scheme:// is encountered. The Hook is called by call/4 using the operation, the IRI and a term that receives the result of the operation. The following operations are defined:

open(Mode,Options)

Called by open/3,4. The result argument must be unified with a stream.

access(Mode)

Called by access_file/2, exists_file/1 (Mode is file) and exists_directory/1 (Mode is directory). The result argument must be unified with a boolean.

time

Called by time_file/2. The result must be unified with a time stamp.

size

Called by size_file/2. The result must be unified with an integer representing the size in bytes.