This package contains an implementation of a tool (pl2curry)
to transform Prolog programs to Curry programs.
The idea of this tool is to demonstrate the advantages of
functional logic languages compared to purely logic languages.
Thus, the tool translates only pure logic programs (without side
effecting predicates etc).
The initial ideas of this tool are described in detail in a
paper presented at ICLP 2022 and published in TPLP:
M. Hanus: From Logic to Functional Logic Programs, Theory and Practice of Logic Programming 22(4), pp. 538-554, DOI: 10.1017/S1471068422000187.
The tool has various options to influence the kind of transformation, e.g.:
--conservative: transform each Prolog predicate into a Curry predicate
Note that this is always possible since non-linear left-hand sides are allowed in Curry (in contrast to Haskell).
with functions (this is the default provided that --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 (this is the default provided that --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 (this is the default provided that --noinline
is not set):
similarly to demand, but bindings are inlined (if possible)
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 2.
A special case of the previous criterion are predicates defined by a single rule, e.g., predicates 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 into 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.
To provide a more reasonable translation to Curry, the translation tool considers also type declarations. These declarations are similarly to polymoprhic algebraic data types but use a Prolog-like syntax. For instance, the Prolog program could contain the directives
:- type nat = o ; s(nat).
:- type tree(A) = leaf(A) ; node(tree(A),tree(A)).These are translated into the Curry type declarations
data Nat = O | S Nat
deriving (Eq,Show)
data Tree a = Leaf a | Node (Tree a) (Tree a)
deriving (Eq,Show)All other constructors occurring in the logic program
(except for true and false and list constructors, see below)
are declared in a single type named Term in Curry.
Note that it is necessary to define type 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,type).
type(_).Due to the fact that Prolog programs are transformed (in the default case)
into nested functions which are lazily evaluated in Curry,
it might be the case that the Curry program computes more answers
than the original Prolog program.
This might be the case if a failing or non-terminating
predicate is evaluated in Prolog but not demanded in the transformed
Curry program. In general, this can be considered as an advantage
of functional logic programming compared to pure logic programming.
However, if it is intended to keep the same answer semantics
between Prolog and the generated Curry programs,
one can specify a set of failing functions (i.e., functions that
are not totally defined due to partial pattern matching or
infinite computations).
In this case, any occurrence of such a failing function will be strictly
evaluated in the Curry program. This fail-sensitvie transformation
will be used if the option --failfuncs=F is provided.
In this case, the file F contains in each line a failing function
in the form Mod f, i.e., f is a function defined in module Mod.
Such files can be generated by the tool curry-calltypes by adding
the option --storefunc (see Curry package verify-non-fail).
The shell script scripts/pl2curry-failsensitive.sh
can be used to apply the fail-sensitive transformation with these tools.
A Docker image with this script and installed versions of all required tools is available as currylang/prolog2curry.
More details about the fail-sensitive transformation can be found in:
M. Hanus: Improving Logic Programs by Adding Functions, Proc. of the 34th International Symposium on Logic-based Program Synthesis and Transformation (LOPSTR 2024), to appear in Springer LNCS, 2024
Examples for this transformation and benchmarks can be found
in the directory benchmarks/LOPSTR2024.
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 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.