Validation API

std.validation provides strongly typed annotation constraints for function parameters and object fields. Parameters are checked before entering the function body; fields are checked before initialization, assignment, and ref<T> writes. Failure throws ConstraintViolation, and the corresponding write is not committed.

import std.validation.CodePointSize
import std.validation.Min
import std.validation.NotBlank

class Account {
  @NotBlank()
  String owner

  @Min(value: 0)
  Integer balance
}

Void register(@CodePointSize(minimum: 3, maximum: 32) String name) {
}

Public contract

All built-in constraints implement Constraint<T>. It combines ParameterInterceptor<T>, FieldInterceptor<T>, and SourceRetention, and defines a constraint through isValid(T), a stable code(), and a display message(). A user annotation implementing the same interface reuses the same lifecycle and failure model.

ConstraintViolation provides:

• location: ConstraintLocation.Parameter or ConstraintLocation.Field;
• functionReference: the Function<?> owning a parameter constraint;
• parameterReference: the Parameter<?> for a parameter constraint;
• fieldReference: the Field<?, ?> for a field constraint;
• code: a stable machine identifier;
• message: the default display text inherited from Exception.

location determines which declaration references are non-null: a Parameter location provides the function and parameter, while a Field location provides the field. Names and types are queried through the references instead of being stored again in the exception.

validation/constraints.norm defines the implementation and complete declarations.

Built-in constraints

Type	Annotations
Boolean	AssertTrue, AssertFalse
Integer	Min, Max, Negative, NegativeOrZero, Positive, PositiveOrZero
String	NotEmpty, NotBlank, CodePointSize, GraphemeSize

CodePointSize and GraphemeSize distinguish Unicode code-point and grapheme-cluster length. Default non-null T and explicitly nullable T? types express nullability constraints.

CodePointSize and GraphemeSize require 0 <= minimum <= maximum. An invalid definition throws ConstraintDefinitionException when the annotation is first executed. Its code provides a stable machine identifier for the corresponding constraint.

Custom constraints

import std.validation.Constraint

annotation Even implements Constraint<Integer> {
  public Boolean isValid(Integer value) {
    return value % 2 == 0
  }

  public String code() {
    return "even"
  }

  public String message() {
    return "must be even"
  }
}

Multiple constraints run in annotation source order and throw on the first failure. A parser that must collect several input errors should form its own structured result before constructing the domain object.