Fix init omitting logic (#163)

* More init doc

* Add "And" utility type

* request bodyが定義されている場合はinitは省略できない

* Fix comment

Author: mpppk

pkgs/docs/docs/04_client/overview.md

@@ -35,7 +35,7 @@ type Spec = DefineApiEndpoints<{
 }>;
 
 const fetchT = fetch as FetchT<"", Spec>;
-const res = await fetchT("/users", {});
+const res = await fetchT("/users");
 const data = await res.json(); // data is { userNames: string[] }
 ```
 
@@ -57,7 +57,7 @@ type Spec = DefineApiEndpoints<{
 }>;
 
 const fetchT = fetch as FetchT<"", Spec>;
-const res = await fetchT("/users", {});
+const res = await fetchT("/users");
 if (!res.ok) {
   // If res.ok is false, status code is 400 or 500
   // So res.json() returns { message: string } | { internalError: string }
@@ -105,10 +105,10 @@ type Spec = DefineApiEndpoints<{
 }>;
 const fetchT = fetch as FetchT<"", Spec>;
 
-await fetchT("/users", {}); // OK
-await fetchT("/users/1", {}); // OK
-await fetchT("/posts", {}); // Error: Argument of type '"/posts"' is not assignable to parameter of type '"/users" | "/users/:id"'.
-await fetchT("/users/1/2", {}); // Error: Argument of type '"/users/1/2"' is not assignable to parameter of type '"/users" | "/users/:id"'.
+await fetchT("/users"); // OK
+await fetchT("/users/1"); // OK
+await fetchT("/posts"); // Error: Argument of type '"/posts"' is not assignable to parameter of type '"/users" | "/users/:id"'.
+await fetchT("/users/1/2"); // Error: Argument of type '"/users/1/2"' is not assignable to parameter of type '"/users" | "/users/:id"'.
 ```
 
 ### Query
@@ -126,9 +126,9 @@ type Spec = DefineApiEndpoints<{
 }>;
 
 const fetchT = fetch as FetchT<"", Spec>;
-await fetchT("/users?page=1", {}); // OK
-await fetchT("/users", {}); // Error: Argument of type string is not assignable to parameter of type MissingQueryError<"page">
-await fetchT("/users?page=1&noexist=1", {}); // Error: Argument of type string is not assignable to parameter of type ExcessiveQueryError<"noexist">
+await fetchT("/users?page=1"); // OK
+await fetchT("/users"); // Error: Argument of type string is not assignable to parameter of type MissingQueryError<"page">
+await fetchT("/users?page=1&noexist=1"); // Error: Argument of type string is not assignable to parameter of type ExcessiveQueryError<"noexist">
 ```
 
 ### headers
@@ -175,6 +175,63 @@ await fetchT("/users", {
 await fetchT("/users", { method: "POST", body: JSONT.stringify({ name: 1 }) }); // Error: Type TypedString<{ userName: number; }> is not assignable to type TypedString<{ userName: string; }>
 ```
 
+### Init
+
+zero-fetch enforces type safety for the `init` parameter of the fetch function. The `init` parameter can be omitted only if all of the following conditions are met:
+
+- The endpoint defines an HTTP GET method.
+- All request headers defined for the endpoint are optional.
+
+If any of these conditions are not satisfied, omitting the `init` parameter will result in a type error.
+
+This behavior ensures that the fetch call adheres strictly to the API specification, preventing runtime errors due to missing or incorrect parameters.
+
+```typescript
+type Spec = DefineApiEndpoints<{
+  "/users": {
+    get: {
+      headers: { "x-api-key"?: string };
+      responses: { 200: { body: { names: string[] } } };
+    };
+  };
+  "/posts": {
+    get: {
+      headers: { "x-api-key": string };
+      responses: { 200: { body: { posts: string[] } } };
+    };
+  };
+}>;
+
+const fetchT = fetch as FetchT<"", Spec>;
+
+await fetchT("/users"); // OK, because GET method is defined and headers are optional
+await fetchT("/users", { headers: { "x-api-key": "key" } }); // OK
+await fetchT("/users", { headers: {} }); // OK, because headers are optional
+await fetchT("/users", { method: "POST" }); // Error: POST method is not defined for this endpoint
+
+await fetchT("/posts"); // Error: "x-api-key" header is required for this endpoint
+await fetchT("/posts", { headers: { "x-api-key": "key" } }); // OK
+```
+
+```typescript
+type Spec = DefineApiEndpoints<{
+  "/posts": {
+    post: {
+      body: { title: string };
+      responses: { 201: { body: { id: string } } };
+    };
+  };
+}>;
+
+const fetchT = fetch as FetchT<"", Spec>;
+
+await fetchT("/posts"); // Error: GET method is not defined for this endpoint
+await fetchT("/posts", {
+  method: "POST",
+  body: JSON.stringify({ title: "New Post" }),
+}); // OK
+```
+
 ## API
 
 ### FetchT

pkgs/typed-api-spec/src/core/type.t-test.ts

@@ -2,6 +2,7 @@ import { Equal, Expect } from "./type-test";
 import {
   AllKeys,
   AllValues,
+  And,
   CountChar,
   ExtractByPrefix,
   FilterNever,
@@ -141,3 +142,17 @@ type AllValuesTestCases = [
   Expect<Equal<AllValues<{ a: 1 } | { a: 2 }, "a">, 1 | 2>>,
   Expect<Equal<AllValues<{ a: 1; b: 3 } | { a: 2 }, "b">, 3>>,
 ];
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+type AndTestCases = [
+  Expect<Equal<And<[]>, true>>, // An empty tuple evaluates to true
+  Expect<Equal<And<[true]>, true>>, // Single element true
+  Expect<Equal<And<[false]>, false>>, // Single element false
+  Expect<Equal<And<[true, true]>, true>>, // All elements are true
+  Expect<Equal<And<[true, false]>, false>>, // Contains false
+  Expect<Equal<And<[false, true]>, false>>, // Contains false
+  Expect<Equal<And<[false, false]>, false>>, // All elements are false
+  Expect<Equal<And<[true, true, true]>, true>>, // Multiple true elements
+  Expect<Equal<And<[true, true, false]>, false>>, // Multiple elements containing false
+  Expect<Equal<And<boolean[]>, boolean>>, // boolean[] type
+];

pkgs/typed-api-spec/src/core/type.ts

@@ -159,3 +159,24 @@ export type AllValues<T, Key extends AllKeys<T>> = T extends {
 }
   ? T[Key]
   : never;
+
+/**
+ * Computes the logical AND of a tuple of boolean types at the type level.
+ * @template T - A readonly tuple of boolean types (true | false | boolean)
+ */
+export type And<T extends readonly boolean[]> = T extends readonly []
+  ? // The AND of an empty tuple is true (identity element of logical AND)
+    true
+  : // Can T be decomposed into [Head, ...Tail]? (Recursive step)
+    T extends readonly [
+        infer Head extends boolean,
+        ...infer Tail extends readonly boolean[],
+      ]
+    ? Head extends false
+      ? // If Head is false, the AND of the entire tuple is false
+        false
+      : // If Head is true, the result depends on the AND of the remaining elements (Tail)
+        And<Tail>
+    : // If T is neither [] nor [Head, ...Tail] (e.g., boolean[] type itself)
+      // In this case, the result cannot be determined and is therefore boolean.
+      boolean;

pkgs/typed-api-spec/src/fetch/index.t-test.ts

@@ -264,6 +264,30 @@ type ValidateUrlTestCase = [
   })();
 }
 
+{
+  type Spec = DefineApiEndpoints<{
+    "/users": {
+      get: {
+        // 本来、GETメソッドはbodyを持たないが、型エラーになることを確認するために定義
+        body: { userName: string };
+        responses: { 200: { body: { prop: string } } };
+      };
+    };
+  }>;
+  (async () => {
+    const f = fetch as FetchT<"", Spec>;
+
+    // @ts-expect-error init cannot be omitted when request body is required
+    await f("/users");
+
+    // Valid case: init is provided with required body
+    await f("/users", {
+      method: "get",
+      body: JSONT.stringify({ userName: "test" }),
+    });
+  })();
+}
+
 {
   type Spec = DefineApiEndpoints<{
     "/packages/list": {

pkgs/typed-api-spec/src/fetch/index.ts

@@ -16,6 +16,7 @@ import {
   ToQueryUnion,
   Method,
   CaseInsensitive,
+  And,
 } from "../core";
 import { UrlPrefixPattern, ToUrlParamPattern } from "../core";
 import { TypedString } from "../json";
@@ -119,8 +120,12 @@ type FetchT<UrlPrefix extends UrlPrefixPattern, E extends ApiEndpoints> = <
    *
    * @template Headers - Request headers object of the endpoint
    *
+   * @template CanOmitHeaders - Whether the headers property in the "init" parameter can be omitted
+   *
    * @template Body - Request body object of the endpoint
    *
+   * @template CanOmitBody - Whether the body property in the "init" parameter can be omitted
+   *
    * @template Response - Response object of the endpoint that matches `CandidatePaths`
    *
    * @template ValidatedUrl - Result of URL validation
@@ -134,7 +139,7 @@ type FetchT<UrlPrefix extends UrlPrefixPattern, E extends ApiEndpoints> = <
    * If the endpoint defines a "get" method, then the method can be omitted
    *
    * @template CanOmitInit - Whether the "init" parameter can be omitted for the request
-   * If the method can be omitted (`CanOmitMethod` is true) and the endpoint does not require headers, then the "init" parameter can be omitted
+   * If the method can be omitted (`CanOmitMethod` is true), headers can be omitted (`CanOmitHeaders` is true), and body can be omitted (`CanOmitBody` is true), then the "init" parameter can be omitted
    */
   UrlPattern extends ToUrlParamPattern<`${UrlPrefix}${keyof E & string}`>,
   Input extends Query extends undefined
@@ -154,7 +159,11 @@ type FetchT<UrlPrefix extends UrlPrefixPattern, E extends ApiEndpoints> = <
   LM extends Lowercase<InputMethod>,
   Query extends ApiP<E, CandidatePaths, LM, "query">,
   Headers extends ApiP<E, CandidatePaths, LM, "headers">,
+  CanOmitHeaders extends Headers extends undefined
+    ? true
+    : IsAllOptional<Headers>,
   Body extends ApiP<E, CandidatePaths, LM, "body">,
+  CanOmitBody extends Body extends undefined ? true : IsAllOptional<Body>,
   Response extends ApiP<
     E,
     CandidatePaths,
@@ -164,22 +173,14 @@ type FetchT<UrlPrefix extends UrlPrefixPattern, E extends ApiEndpoints> = <
     ? MergeApiResponseBodies<ApiP<E, CandidatePaths, LM, "responses">>
     : Record<StatusCode, never>,
   ValidatedUrl extends ValidateUrl<Query, Input>,
+  CanOmitMethod extends "get" extends AcceptableMethods ? true : false,
   InputMethod extends CaseInsensitive<AcceptableMethods> = Extract<
     AcceptableMethods,
     "get"
   >,
-  CanOmitMethod extends boolean = "get" extends AcceptableMethods
-    ? true
-    : false,
-  CanOmitInit extends boolean = CanOmitMethod extends true
-    ? Headers extends undefined
-      ? true
-      : Headers extends Record<string, string>
-        ? IsAllOptional<Headers> extends true
-          ? true
-          : false
-        : false
-    : false,
+  CanOmitInit extends boolean = And<
+    [CanOmitMethod, CanOmitHeaders, CanOmitBody]
+  >,
 >(
   input: [ValidatedUrl] extends [C.OK | QueryParameterRequiredError]
     ? Input