Curry: An Integrated Functional Logic Language

functions must be free of side-effects in order to be compatible with Curry’s computation model.
An external constraint solver consists of a constraint store which can be accessed and manipulated
via a few primitive operations.
9.1 External Functions
External functions are considered as an implementation of a potentially infinite set of equations
(describing the graph of the functions). In particular, they have no side effects, i.e., identical
function calls at different points of time yield identical results. Since the implementation of an
external function is unknown, a call to an external function is suspended until all arguments are
fully known, i.e., until they are evaluated to ground data terms. This view is compatible with the
residuation principle covered by Curry’s computation model and very similar to the connection of
external functions to logic programs [9, 10].
An external function is declared by a type declaration followed by external(FName,OF,Lang)
where FName is the name of the external function where its code is contained in the object file OF.
Lang is the implementation language of the external function. For instance, if the addition and
multiplication on integers are defined as C functions named add and mult where the compiled code
is contained in the file “arith.o”, we can connect these functions to a Curry program by providing
the declarations
add :: Int -> Int -> Int external("add","arith.o",C)
mult :: Int -> Int -> Int external("mult","arith.o",C)
Implementations of Curry might have restrictions on the interface. A reasonable requirement is
that the implementation language is constrained to C and the argument and result types are only
simple types like Bool, Int, Float, Char, or String.
9.2 External Constraint Solvers
A constraint solver can be viewed as an abstract datatype consisting of a constraint store together
with a few operations to check the entailment of a constraint or to add a new constraint. In order
to connect a constraint solver to Curry, like a solver for arithmetic or Boolean constraints, the
external solver must provide the following operations (cs denotes the type of the constraint store
and c the type of constraints):
new : → cs (create and initialize a new constraint store)
ask : cs × c → {T rue, F alse, Unknown} (check entailment of a constraint)
tell : cs × c → cs (add a new constraint)
clone : cs → cs × cs (copy the constraint store)
Using these operations, it is relatively easy to extend Curry’s computation model to include constraints
by adding the constraint store as a new component to the substitution part in answer
expressions (cf. Section 3). 12
12 This section will be modified or extended in a later version.
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