Basic Concepts¶

This section introduces the fundamental concepts of the constraint_handler library. It explains how to define data, perform calculations, and enforce rules within your ASP programs.

For a complete specification of every predicate and operator, please consult the Reference section. It's recommended to start with the Language Concepts to better understand the overall design philosophy.

Input & Output¶

The constraint handler operates on a simple principle:

Input: You write declarative rules using the handler's predicates and function symbols (like variable_define, ensure, operation) to define your problem.
Output: The handler processes these rules and adds new atoms (like value) to the final stable model, representing the computed results.

Types¶

One of the main advantages of the constraint handler is its ability to seamlessly manage data types that are not natively supported by standard ASP solvers.

While Clingo primarily handles integers and symbolic constants, the constraint handler extends this to include:

Primitives: Floats, Strings, and Bools.
Collections: Sets and Multimaps.

For a comprehensive list of supported types, please refer to the Base Types and Collections sections in the reference.

Values¶

Because the handler supports many types, it needs a way to distinguish between them (e.g., the integer 5 vs. the string "5"). We use the val/2 wrapper for this purpose.

Syntax: val(Type, Data)

Example

val(int, 42)
val(string, "Hello World")
val(float, float("3.14"))

For full details, see the Value reference.

Variables & Assignment¶

Variables allow you to store specific values and reuse them later. You can create them using the variable_define/2 predicate.

Syntax: variable_define(Name, Expression)

Name: The name you will use to refer to this data.
Expression: The value or calculation to assign.

Example

Assigning the integer 42 to a variable named x:

variable_define(x, val(int, 42)).

To use this variable in a later Expression, you reference it using variable(x).

For full details, see the Variable reference.

Operations¶

To perform calculations—such as arithmetic, logical comparisons, or set manipulations—you use Operations.

Syntax: operation(Operator, Arguments)

Operator: The specific action (e.g., add, mult, union, eq).
Arguments: A recursive list of inputs.

Example

Adding two variables x and y:

operation(add, (variable(x), (variable(y), ())))

For more information, refer to the Operation reference. A list of operators is attached to each respective type in the Base Type or Collections sections.

Constraints¶

While assignments create data, constraints validate it.

The ensure/1 predicate asserts that a specific condition must be true. If the condition fails, the constraint handler rejects the current model (similar to an integrity constraint :- ... in standard ASP).

Syntax: ensure(Condition)

Example

Ensuring that variable z is greater than 10:

ensure(operation(gt, (variable(z), (val(int, 10), ())))).

For more details, see Ensure in the reference.