Errors and exceptions

The Prolog language has exceptions. 2P-Kt's design aims at defining them in the implementation language with a one-to-one mapping.

In 2P-Kt, exceptions are divided into three main categories, which will be better described in the following sections:

• Prolog-level errors, as defined by the ISO standard;
• Run-time exceptions, thrown at state-machine level during the resolution process;
• 2P-Kt exceptions, thrown at the implementation language level.

2P-Kt exceptions

Exceptions in 2P-Kt are all implemented by extending the TuPrologException class, which in turn extends Kotlin's RuntimeException.

This type defines two properties:

• message: String? the detailed error message,
• cause: Throwable? the cause of the exception.

Run-time exceptions

Run-time exceptions happen during Solver executions. They are defined by TuPrologException's subclass TuPrologRuntimeException which adds the contexts: Array<ExecutionContext> property, describing the stack of contexts that localize the exception in the execution flow.

Prolog-level errors

These kind of errors occur when something goes wrong at Prolog language level. Their definitions and properties are described by an ISO standard, which explains their meaning and when they should be raised. Each Prolog implementation is expected to support them, and 2P-Kt strives to do so.

In 2P-Kt, these errors are defined as subclasses of the PrologError type, which extends TuPrologRuntimeException. PrologError defines two extra properties:

• type: Struct describing the error structure,
• extraData: Term? which can contain any arbitrary implementation-dependant data.

PrologError instances can be created using its companion object factory method of method, which creates the correct instance of the error according to the given type structure. If it cannot recognise a corrisponding error, an anonymous PrologError instance is created instead.

Supported errors

At the time being, 2P-Kt supports the following Prolog errors:

• DomainError occurs when something has the correct type but the value is not admissible
• EvaluationError occurs when some problem occurs in evaluating an arithmetic expression
• ExistenceError occurs when an object on which an operation is to be performed does not exist
• InstantiationError occurs when some Term is a Variable, and it should not
• MessageError used whenever no other PrologError instance is suitable for representing the error
• PermissionError occurs when an attempt to perform a prohibited operation is made
• RepresentationError occurs when an implementation limit has been reached
• SyntaxError occurs when there is a parsing failure
• SystemError occurs when an internal problem has occurred; if it is not caught, it will halt inferential machine
• TypeError occurs when something is not of Expected type