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Curry Package prolog2curry

prolog2curry - Transforming Prolog programs to Curry programs

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.

Tool options

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).
  • 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:

  • If 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


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.

Type declarations:

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).

Fail-sensitive transformation:

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.

Technical remarks:

  • 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.


Checkout with CPM:
cypm checkout prolog2curry 1.2.0
Package source:
prolog2curry-1.2.0.tar.gz [browse]
Source repository: