feat(core): add ExtractFormValues type (#45)

- add ExtractFormValues type
- make FormSchema into nominal type

Co-authored-by: Mikołaj Klaman <mklaman@virtuslab.com>

src/core/helpers/create-initial-values.spec.ts

@@ -4,7 +4,9 @@ import { createInitialValues } from "./create-initial-values";
 
 describe("createInitialValues", () => {
   it("should be empty for empty schema", () => {
-    expect(createInitialValues({})).toEqual({});
+    const schema = createFormSchema(() => ({}));
+
+    expect(createInitialValues(schema)).toEqual({});
   });
 
   it("for one-element schema", () => {

src/core/helpers/create-initial-values.ts

@@ -9,7 +9,7 @@ export const createInitialValues = <Values extends object>(
 ): Values => {
   const initialStateFromDecoders = entries(schema).reduce(
     (shape, [key, descriptor]) => {
-      shape[key] = impl(descriptor).__decoder.init();
+      shape[key] = impl(descriptor).__decoder.init() as Values[keyof Values];
       return shape;
     },
     {} as Values

src/core/index.ts

@@ -13,6 +13,6 @@ export { FormtsOptions } from "./types/formts-options";
 export { FieldDecoder } from "./types/field-decoder";
 export { FieldDescriptor } from "./types/field-descriptor";
 export { FieldHandle } from "./types/field-handle";
-export { FormSchema } from "./types/form-schema";
+export { FormSchema, ExtractFormValues } from "./types/form-schema";
 export { FormValidator, Validator } from "./types/form-validator";
 export { FormController } from "./types/form-controller";

src/core/types/field-decoder.ts

@@ -49,7 +49,7 @@ export const isArrayDecoder = <T>(
  * Should be used together with `createForm.schema` function.
  */
 // @ts-ignore
-export interface FieldDecoder<T> extends Nominal<"FieldDecoder", {}> {}
+export interface FieldDecoder<T> extends Nominal<"FieldDecoder"> {}
 
 export type FieldType =
   | "number"

src/core/types/field-descriptor.ts

@@ -20,7 +20,7 @@ export type _NTHHandler<T> = {
  */
 // @ts-ignore
 export interface FieldDescriptor<T, Err = unknown>
-  extends Nominal<"FieldDescriptor", {}> {}
+  extends Nominal<"FieldDescriptor"> {}
 
 // prettier-ignore
 export type GenericFieldDescriptor<T, Err = unknown> =

src/core/types/form-controller.ts

@@ -5,7 +5,7 @@ import { InternalFormtsContext } from "./formts-context";
 /**
  * Used to connect together hooks or FormProvider component with the actual form state
  */
-export interface FormController extends Nominal<"FormControl", {}> {}
+export interface FormController extends Nominal<"FormControl"> {}
 
 export type _FormControllerImpl<Values extends object, Err> = {
   __ctx: InternalFormtsContext<Values, Err>;

src/core/types/form-schema.spec.ts

@@ -1,7 +1,7 @@
 import { IsExact, assert } from "conditional-type-checks";
 
 import { FieldDescriptor } from "./field-descriptor";
-import { FormSchema } from "./form-schema";
+import { FormSchema, ExtractFormValues } from "./form-schema";
 
 type SomeValues = {
   string: string;
@@ -158,3 +158,21 @@ describe("FormSchema type", () => {
     assert<IsExact<Actual, Expected>>(true);
   });
 });
+
+describe("ExtractFormValues type", () => {
+  it("extracts type of form values out of FormSchema type", () => {
+    type Schema = FormSchema<SomeValues, SomeErr>;
+
+    type Actual = ExtractFormValues<Schema>;
+    type Expected = SomeValues;
+
+    assert<IsExact<Actual, Expected>>(true);
+  });
+
+  it("resolves to never for invalid input", () => {
+    type Actual = ExtractFormValues<{}>;
+    type Expected = never;
+
+    assert<IsExact<Actual, Expected>>(true);
+  });
+});

src/core/types/form-schema.ts

@@ -1,9 +1,16 @@
+import { Nominal } from "../../utils";
+
 import { GenericFieldDescriptor } from "./field-descriptor";
 
 /**
  * Description of a form.
  * Used to interact with Formts API and point to specific form fields.
  */
-export type FormSchema<Values extends object, Err> = {
-  readonly [K in keyof Values]: GenericFieldDescriptor<Values[K], Err>;
-};
+export type FormSchema<Values extends object, Err> = Nominal<"FormSchema"> &
+  {
+    readonly [K in keyof Values]: GenericFieldDescriptor<Values[K], Err>;
+  };
+
+export type ExtractFormValues<Schema> = Schema extends FormSchema<infer V, any>
+  ? V
+  : never;

src/utils/utility-types.ts

@@ -1,8 +1,7 @@
-class Tagged<T> {
+export class Nominal<Tag> {
   // @ts-ignore
-  private __tag!: T;
+  private __tag!: Tag;
 }
-export type Nominal<Tag, T> = Tagged<Tag> & T;
 
 export type UnionToIntersection<U> = (
   U extends any ? (k: U) => void : never