Language.Prolog.Read
This package contains an implementation of a tool (pl2curry
) to transform Prolog programs to Curry programs. The tool has various options to influence the kind of transformation, e.g.:
--conservative
: transform each Prolog predicate into a Curry predicateNote that this is always possible since non-linear left-hand sides are allowed in Curry (in contrast to Haskell).
with functions (default, i.e., --conservative
is not set): specify some predicates as functions by:
:- function p/n.
: The last argument is the result.:- function p/n: i.
: The i-th argument is the result (i=n: last argument).:- function p/n: [i1,...,ik].
: Argument positions [i1,...,ik] are put as result arguments.
For instance, if p/3
is a function and the last argument is the result, then a goal p(X,Y,Z)
is transformed into z =:= p x y
.
Note that it is necessary to define function
as an operator in Prolog in order to read Prolog programs with such directives. This can be done by adding the following directives at the beginning of the Prolog program:
:- op(1150,fx,function).
function(_).
with demand (default, i.e., --nodemand
is not set): similarly to functions, but function calls are transformed into local variable bindings rather than unifications. Hence, functions are evaluated only if its result is demanded (due to Curry's lazy evaluation strategy).
with inlining (default, i.e., --noinline
is not set): similarly to demand, but bindings are inlined to obtain a more compact source code
Since adding function
directives to specify result argument positions is tedious, the tool also contains an analysis to derive automatic function
directives (if not already explicitly provided and if the option --noanalysis
is not set) for standard functions (i.e., where only the last argument is a result position). It is based on the following principle:
p
is an n-ary predicate and there is a (minimal) set of argument position such that the rules for p
are inductively sequentially defined on this set of argument positions (in particular, non-overlapping w.r.t. these arguments), p
is considered as a function (where the last argument is the result argument position, or the maximum of the remaining argument positions if the option --anyresult
is set).The information about the inferred sets of inductively sequential argument and result arguments of functions is printed if the verbosity is larger than 1.
A special case of the previous criterion are predicates defined by a single rule, e.g., predicate which define constants, as
two(s(s(o))).
This will be translated into
two = S (S O)
Although this is correct, it is sometimes unintended, e.g., if p
is defined by the single clause
p(X,Y) :- q(X,Z), r(Z,Y).
In order to keep such predicates as predicates on the Curry level, the following heuristic is used. A predicate defined by a single rule is transformed as a function only if the last argument in the left-hand side is not a variable or a variable which occurs in a result argument position in the rule's body.
Although these heuristics provide expected transformations in most case, one can always override them using an explicit function
directive.
Prolog atoms true
and false
are translated into the Curry Boolean constants True
and False
.
Prolog lists are transformed into Curry lists (unless the option --nolists
is set). This should yield the intended code but might produce type errors for strange uses of Prolog lists, e.g., .(1,.(2,3))
. If Curry lists are not used (by setting the option --nolists
), Prolog lists are transformed into Curry terms by using the constructors NIL
and CONS
, e.g., the Prolog list [1,2]
is transformed into CONS 1 (CONS 2 NIL)
.
In the default case, only the last argument of a predicate will be considered as a result argument position for inferred functions. Hence, if the last argument is a contained in a minimal set of inductively sequential arguments, it will not be transformed into a function.
This behavior can be changed by setting the option --anyresult
. In this case, a maximum argument will be selected. If this is not the last one, the index of the result argument position is added to indicate that the order of arguments has been changed in the transformed function. See examples/demand.pl
for such examples.