Discussions with Jeff Schultz helped shaping this library
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
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:
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.
We distinguish two views on options. One is to see them as additional
parameters that require strict existence, type and domain-checking and
the other is to consider them `locally scoped environment variables'.
Most systems adopt the first option. SWI-Prolog adopts the second: it
silently ignores options that are not supported but does type and domain
checking of option-values. The `environment' view is commonly used in
applications to create predicates supporting more options using the
skeleton below. This way of programming requires that pred1 and
pred2 do not interpret the same option differently. In cases
where this is not true, the options must be distributed by some_pred.
We have been using this programming style for many years and in practice
it turns out that the need for active distribution of options is rare.
I.e., options either have distinct names or multiple predicates
implement the same option but this has the desired effect. An example of
the latter is the encoding option, which typically needs to
be applied consistently.
some_pred(..., Options) :-
pred1(..., Options),
pred2(..., Options).
As stated before, options provide a readable alternative to high-arity predicates and offer a robust mechanism to evolve the API, but at the cost of some runtime overhead and weaker consistency checking, both at compiletime and runtime. From our experience, the `environment' approach is productive, but the consequence is that mistyped options are silently ignored. The option infrastructure described in this section tries to remedy these problems.
Whether we see options as arguments or locally scoped environment variables, the most obvious way to improve on the current situation is to provide reflective support for options: discover that an argument is an option-list and find what options are supported. Reflective access to options can be used by the compiler and development environment as well as by the runtime system to warn or throw errors.
An obvious approach to deal with options is to define the different possible option values as a type and type the argument that processes the option as list(<option_type>), as illustrated below. Considering options as types fully covers the case where we consider options as additional parameters.
:- type open_option ---> type(stream_type) |
alias(atom) | ... .
:- pred open(source_sink, open_mode, stream, list(open_option)).
There are three reasons for considering a different approach:
From the above, we conclude that we require reflective access to find
out whether an option is supported and valid for a particular predicate.
Possible option values must be described by types. Due to lack of a type
system, we use library(error) to describe allowed option
values. Predicate options are declared using predicate_options/3:
Below is an example that processes the option header(boolean) and passes all options to open/4:
:- predicate_options(write_xml_file/3, 3,
[ header(boolean),
pass_to(open/4, 4)
]).
write_xml_file(File, XMLTerm, Options) :-
open(File, write, Out, Options),
( option(header(true), Option, true)
-> write_xml_header(Out)
; true
),
...
This predicate may only be used as a directive and is processed by expand_term/2. Option processing can be specified at runtime using assert_predicate_options/3, which is intended to support program analysis.
false, the predicate becomes semidet and
fails without modifications if modifications are required.
The predicates below realise the support for compile and runtime checking for supported options.
?- current_predicate_option(open/4, 4, type(text)). true.
This predicate is intended to support conditional compilation using if/1 ... endif/0. The predicate current_predicate_options/3 can be used to access the full capabilities of a predicate.
The predicates below can be used in a development environment to inform the user about supported options. PceEmacs uses this for colouring option names and values.
The library can execute a complete check of your program using check_predicate_options/0:
The library offers predicates that may be used to create declarations for your application. These predicates are designed to cooperate with the module system.
library(option) or passes options to other
predicates that are known to process options. The process is repeated
until no new declarations are retrieved.
current_output stream.