Assignability

Assignability determines whether a source type is assignable to a target type or not. Assignability is used for the following use cases, among others:

• Assignment statements, e.g. targetVariable := sourceValue;: Assignability indicates whether the type of the given source value type-wise matches the type of the target variable.
• Variable initialization, e.g. var newVariable: VariableType := initialValue;: Assignability indicates whether the type of the initial value matches the explicitly declared type of the new variable.
• Calling functions and operators with arguments, e.g. myFunction(inputArgument): Assignability indicates whether the type of a given arguments matches the expected type of the corresponding parameter.

API

The assignability service informs, whether a source type is assignable to a target type:

typir.Assignability.isAssignable(source: Type, target: Type): boolean

Default implementation

The default implementation exploits the following relationships between types:

• equality
• implicit conversion (but no explicit conversion / casting)
• sub-type relationships

Result of assignability: Chain of assignability

If you are interested in more details, why types are assignable or why two types are not assignable, use the following more advanced API to get a result object, which gives some more information:

typir.Assignability.getAssignabilityResult(source: Type, target: Type): AssignabilityResult
 
type AssignabilityResult = AssignabilitySuccess | AssignabilityProblem;
 
interface AssignabilitySuccess {
    //...
    result: true;
    path: Array<SubTypeEdge | ConversionEdge>;
}
 
interface AssignabilityProblem extends TypirProblem {
    //...
    result: false;
    subProblems: TypirProblem[];
}

In case of no assignability, you will get an AssignabilityProblem, whose subProblems might list some found problems.

In case of assignability, you will get an AssignabilitySuccess, which contains a path of edges found in the type graph, which starts at the source type and ends at the target type. The edges represent equality, an existing sub-type relationship or an implicit conversion between the current type to the next type. If source type and target type are the same, an empty path is returned.