Error visitor for custom types

[laurence-myers]

Hello! 👋🙂

In my app, I have implemented a "discriminated union" custom type, and registered it in TypeRegistry. I discovered that TypeBox's error reporting for custom types ("Kind") is limited, and only says "Expected NameOfCustomType". I could specify a custom error message, using SetErrorFunction(), but the info available at that point is very limited.

With a bit of tinkering, I found that TypeBox could potentially support a custom "visitor" as part of Errors()! The visitor would be a generator function, yielding one or more error ValueErrors.

I was wondering if I could add this to TypeBox, and if so, how you would like it to look?

For a proper implementation, I was thinking of changing TypeRegistry to either accept a function, or an object, roughly like this:

type TypeRegistryValidationFunction<TSchema> = (schema: TSchema, value: unknown) => boolean;

interface TypeRegistryEntry<TCustomSchema> {
  validate(schema: TCustomSchema, value: unknown): boolean;
  visit?(schema: TCustomSchema, references: TSchema[], path: string, value: unknown): Generator<ValueError>;
}

interface TypeRegistry {
  Set<TSchema = unknown>(kind: string, funcOrObj: TypeRegistryValidationFunction<TSchema> | TypeRegistryEntry<TSchema>): void
}

For my app, I've just used patch-package to add a quick hack to TypeBox.

(Click to see the patch)

index 7525b75..6a2324d 100644
--- a/node_modules/@sinclair/typebox/build/esm/errors/errors.mjs
+++ b/node_modules/@sinclair/typebox/build/esm/errors/errors.mjs
@@ -506,8 +506,14 @@ function* FromVoid(schema, references, path, value) {
 }
 function* FromKind(schema, references, path, value) {
     const check = TypeRegistry.Get(schema[Kind]);
-    if (!check(schema, value))
-        yield Create(ValueErrorType.Kind, schema, path, value);
+    if (!check(schema, value)) {
+        // Support custom visitor for a custom type
+        if (check.Visit) {
+            yield* check.Visit(schema, references, path, value);
+        } else {
+            yield Create(ValueErrorType.Kind, schema, path, value);
+        }
+    }
 }
 function* Visit(schema, references, path, value) {
     const references_ = IsDefined(schema.$id) ? [...references, schema] : references;

Usage (cut down from my actual implementation, but gives you an idea):

TypeRegistry.Set(
  'DiscriminatedUnion',
  (function (): TypeRegistry.TypeRegistryValidationFunction<TSchema> {
    const validationFunction = function (schema: TSchema, value: any): boolean {
      for (const subSchema of schema.allOf) {
        if (Check(subSchema.if, value)) {
          return Check(subSchema.then, value)
        }
      }

      return false;
    }

    validationFunction.Visit = function* (schema: TSchema, references: TSchema[], path: string, value: unknown) {
      for (const subSchema of schema.allOf) {
        if (Check(subSchema.if, value)) {
          for (const error of Errors(subSchema.then, references, value)) {
            yield {
              ...error,
              path: path + error.path,
            };
          }
        }
      }
    }

    return validationFunction;
  })()
);