Interface ParameterDefaultValueProvider

All Superinterfaces:
Ordered
All Known Implementing Classes:
PythonParameterDefaultValueProvider
public interface ParameterDefaultValueProvider extends Ordered
Supplies the declared default value of a ParameterElement as an expression the caller evaluates at the invocation site.

Implementations are discovered with the service loader, so a language module contributes one without core knowing about the language. Each is asked supports(ParameterElement) before use, in Ordered order, and the first that supplies an expression wins.

This supports languages that compile a default argument to an accessor the caller invokes. Scala, for example, compiles class Greeter(greeting: String = "hello") to a single constructor plus a static $lessinit$greater$default$1() getter, so an absent argument can be filled in like:

new Greeter(greeting != null ? greeting : Greeter.$lessinit$greater$default$1())

It is deliberately not a general mechanism for default arguments. Languages that evaluate defaults inside the callee cannot use it — a Kotlin default may reference this and preceding parameters and exists only within the synthetic $default body (see KotlinParameterElement), and an interpreted language's default may not be expressible as bytecode at all. Such languages report ParameterElement.hasDefault() without contributing a provider, and supply the value by their own means.

Since:
5.2.0
Author:
graemerocher

  • Field Summary

    Fields inherited from interface Ordered

    HIGHEST_PRECEDENCE, LOWEST_PRECEDENCE

  • Method Summary

    Modifier and Type
    Method
    Description
    Optional<io.micronaut.sourcegen.model.ExpressionDef>
    defaultValueExpression(ParameterElement parameter, @Nullable io.micronaut.sourcegen.model.ExpressionDef target)
    Supplies an expression evaluating to the parameter's declared default value.
    boolean
    supports(ParameterElement parameter)
    Whether this provider can supply default values for the given parameter, which is typically a test of the element's language specific implementation type.

    Methods inherited from interface Ordered

    getOrder

  • Method Details

    • supports

      boolean supports(ParameterElement parameter)
      Whether this provider can supply default values for the given parameter, which is typically a test of the element's language specific implementation type.
      Parameters:
      parameter - The parameter
      Returns:
      True if defaultValueExpression(ParameterElement, ExpressionDef) should be consulted for this parameter

    • defaultValueExpression

      Optional<io.micronaut.sourcegen.model.ExpressionDef> defaultValueExpression(ParameterElement parameter, @Nullable io.micronaut.sourcegen.model.ExpressionDef target)
      Supplies an expression evaluating to the parameter's declared default value.

      The returned expression must be assignable to the erasure of ParameterElement.getType() and must be safe to evaluate at most once per invocation, in argument position. Implementations should be side effect free, as this method may be called more than once for the same parameter.

      Parameters:
      parameter - The parameter, for which ParameterElement.hasDefault() is true
      target - The receiver of the call the parameter belongs to, or null when the default is obtained without one — as for a constructor, or where the language emits a static accessor
      Returns:
      The expression, or Optional.empty() if the default cannot be materialised at the call site, in which case the caller falls back to the default value of the parameter's type