Many ISO predicates accept options, e.g., open/4, write_term/3.
Options offer an attractive alternative to proliferation into many
predicates and using high-arity predicates. Properly defined and used,
they also form a mechanism for extending the API of both system and
application predicates without breaking portability. I.e., previously
fixed behaviour can be replaced by dynamic behaviour controlled by an
option where the default is the previously defined fixed behaviour. The
alternative to using options is to add an additional argument and
maintain the previous definition. While a series of predicates with
increasing arity is adequate for a small number of additional
parameters, the untyped positional argument handling of Prolog quickly
makes this unmanageable.
The ISO standard uses the extensibility offered by options by
allowing implementations to extend the set of accepted options. While
options form a perfect solution to maintain backward portability in a
linear development model, it is not well equipped to deal with
concurrent branches because
- There is no API to find which options are supported in a particular
implementation.
- While the portability problem caused by a missing predicate in
Prolog A can easily be solved by implementing this predicate, it
is much harder to add processing of an additional option to an already
existing predicate.
Different Prolog implementations can be seen as concurrent
development branches of the Prolog language. Different sets of supported
options pose a serious portability issue. Using an option O that
establishes the desired behaviour on system A leads (on most
systems) to an error or system B. Porting may require several
actions:
- Drop O (if the option is not vital, such as the layout
options to write_term/3)
- Replace O by O2 (i.e., a differently named option
doing the same)
- Something else (cannot be ported; requires a totally different
approach, etc.)
Predicates that process options are particularly a problem when
writing a compatibility layer to run programs developed for System A
on System
B because complete emulation is often hard, may cause a serious
slowdown and is often not needed because the application-to-be-ported
only uses options that are shared by all target Prolog implementations.
Unfortunately, the consequences of a partial emulation cannot be
assessed by tools.
