feat(fetch): add result-based SafeFetch wrapper [Codex]

docs/content/docs/api-surface.mdx

@@ -113,6 +113,70 @@ let response = await WebAPI.Fetch.fetch(
 let text = await response->WebAPI.Response.text
 ```
 
+### Result-based workflows
+
+`WebAPI.Fetch` and `WebAPI.Response` are the raw browser bindings. Use the opt-in
+`WebAPI.SafeFetch` helpers when request, HTTP status, and body-reading failures should be handled as
+`result` values. The wrapper keeps those failure stages separate and preserves the native values
+needed to inspect each outcome.
+
+| Stage                   | Result                             | Preserved context               |
+| ----------------------- | ---------------------------------- | ------------------------------- |
+| Fetch throws or rejects | `Error(FetchRejected(cause))`      | Original rejection as `exn`     |
+| Response is not OK      | `Error(ResponseNotOk({response}))` | Complete response               |
+| Body reader rejects     | `Error({response, cause})`         | Response and original rejection |
+
+`SafeFetch.fetch` accepts a URL, while `SafeFetch.fetchWithRequest` accepts an existing
+`WebAPI.Request.t`. Both promises fulfill with a `result` for these expected failures. A fulfilled
+response is classified with the platform's `response.ok` value, so a 404 or 500 is distinct from a
+request that did not produce a response.
+
+```ReScript
+switch await WebAPI.SafeFetch.fetch("https://example.com/api/users") {
+| Error(WebAPI.SafeFetch.FetchRejected(cause)) => handleUnavailable(cause)
+| Error(WebAPI.SafeFetch.ResponseNotOk({response})) => handleHttpError(response.status)
+| Ok(response) =>
+  switch await response->WebAPI.SafeFetch.json {
+  | Error({response, cause}) => handleInvalidBody(response.status, cause)
+  | Ok({response, body}) => handleJson(response.headers, body)
+  }
+}
+```
+
+Use `checkOk` to classify a response obtained from the raw API without reading its body.
+
+```ReScript
+let response = await WebAPI.Fetch.fetch("https://example.com/api/users")
+
+switch response->WebAPI.SafeFetch.checkOk {
+| Ok(response) => useResponse(response)
+| Error({response}) => handleHttpError(response.status)
+}
+```
+
+A non-OK response remains readable. This is useful when a server returns structured error details.
+
+```ReScript
+switch await WebAPI.SafeFetch.fetch("https://example.com/api/users/42") {
+| Error(WebAPI.SafeFetch.ResponseNotOk({response})) =>
+  switch await response->WebAPI.SafeFetch.text {
+  | Ok({body}) => Console.error(body)
+  | Error(_) => Console.error(`Request failed with ${Int.toString(response.status)}`)
+  }
+| Error(WebAPI.SafeFetch.FetchRejected(cause)) => reportUnavailable(cause)
+| Ok(response) => useResponse(response)
+}
+```
+
+The safe body readers cover `arrayBuffer`, `blob`, `bytes`, `formData`, `json`, and `text`. A
+successful read contains both the body and original response, preserving status, headers, URL, and
+other metadata. Reading consumes the native body; clone the response before reading when another
+consumer also needs it.
+
+`SafeFetch.json` returns `JSON.t`; validating that JSON against an application's domain types remains
+the application's responsibility. This keeps malformed JSON text, which becomes a `readError`,
+separate from valid JSON that does not match an application schema.
+
 `BodyInit` has constructors for the common JavaScript body shapes.
 
 ```ReScript

rescript.json

@@ -330,7 +330,8 @@
         "Headers",
         "HeadersInit",
         "Request",
-        "Response"
+        "Response",
+        "SafeFetch"
       ]
     },
     {

src/fetch/SafeFetch.res

@@ -0,0 +1,57 @@
+type httpError = {
+  response: Response.t,
+}
+
+type fetchError =
+  | FetchRejected(exn)
+  | ResponseNotOk(httpError)
+
+type readError = {
+  response: Response.t,
+  cause: exn,
+}
+
+type response<'body> = {
+  response: Response.t,
+  body: 'body,
+}
+
+let checkOk = (response: Response.t): result<Response.t, httpError> =>
+  if response.ok {
+    Ok(response)
+  } else {
+    Error({response: response})
+  }
+
+let fetch = async (url: string, ~init: option<Request.requestInit>=?) => {
+  try {
+    let response = await SafeFetchRaw.fromUrl(url, ~init?)
+    response->checkOk->Result.mapError(error => ResponseNotOk(error))
+  } catch {
+  | cause => Error(FetchRejected(cause))
+  }
+}
+
+let fetchWithRequest = async (request: Request.t, ~init: option<Request.requestInit>=?) => {
+  try {
+    let response = await SafeFetchRaw.fromRequest(request, ~init?)
+    response->checkOk->Result.mapError(error => ResponseNotOk(error))
+  } catch {
+  | cause => Error(FetchRejected(cause))
+  }
+}
+
+let read = async (response: Response.t, readBody: Response.t => promise<'body>) => {
+  try {
+    Ok({response, body: await readBody(response)})
+  } catch {
+  | cause => Error({response, cause})
+  }
+}
+
+let arrayBuffer = response => read(response, Response.arrayBuffer)
+let blob = response => read(response, Response.blob)
+let bytes = response => read(response, Response.bytes)
+let formData = response => read(response, Response.formData)
+let json = response => read(response, Response.json)
+let text = response => read(response, Response.text)

src/fetch/SafeFetch.resi

@@ -0,0 +1,114 @@
+/**
+Describes a response whose `ok` field is `false`.
+
+The original response is retained so callers can inspect its status, headers, and body.
+*/
+type httpError = {
+  response: Response.t,
+}
+
+/**
+Describes an expected failure while producing or classifying a response.
+
+- `FetchRejected(cause)` means the native Fetch operation threw or its promise rejected. Fetch does
+  not expose a reliable browser-independent distinction between network, abort, permission, and
+  other request failures.
+- `ResponseNotOk({response})` means Fetch fulfilled with a response whose `ok` field is `false`.
+  The response remains available for inspection and body reading.
+*/
+type fetchError =
+  | FetchRejected(exn)
+  | ResponseNotOk(httpError)
+
+/**
+Describes a rejected response body reader.
+
+The record retains both the response whose body was being consumed and the original rejection
+cause.
+*/
+type readError = {
+  response: Response.t,
+  cause: exn,
+}
+
+/**
+A successfully read body paired with its original response.
+
+The response body has been consumed. Clone the response before reading when another consumer also
+needs the body.
+*/
+type response<'body> = {
+  response: Response.t,
+  body: 'body,
+}
+
+/**
+Classifies an existing response using its native `ok` field.
+
+Returns `Ok(response)` for successful responses and `Error({response})` otherwise. This function
+does not read the body and is useful with responses obtained from the raw `Fetch` API.
+*/
+let checkOk: Response.t => result<Response.t, httpError>
+
+/**
+Fetches a URL and returns expected request and HTTP status failures as values.
+
+The returned promise fulfills with `Error(FetchRejected(cause))` when the native operation throws or
+rejects, `Error(ResponseNotOk({response}))` when the response is not OK, or `Ok(response)` for an OK
+response.
+*/
+let fetch: (string, ~init: Request.requestInit=?) => promise<result<Response.t, fetchError>>
+
+/**
+Fetches an existing `Request.t` and returns expected request and HTTP status failures as values.
+
+This has the same result contract as `fetch` and accepts an optional request initializer that is
+passed to the native Fetch operation.
+*/
+let fetchWithRequest: (
+  Request.t,
+  ~init: Request.requestInit=?,
+) => promise<result<Response.t, fetchError>>
+
+/**
+Reads the response body as an `ArrayBuffer.t`.
+
+Returns the body with the original response, or a `readError` when the native reader rejects.
+*/
+let arrayBuffer: Response.t => promise<result<response<ArrayBuffer.t>, readError>>
+
+/**
+Reads the response body as a `Blob.t`.
+
+Returns the body with the original response, or a `readError` when the native reader rejects.
+*/
+let blob: Response.t => promise<result<response<Blob.t>, readError>>
+
+/**
+Reads the response body as an array of bytes.
+
+Returns the body with the original response, or a `readError` when the native reader rejects.
+*/
+let bytes: Response.t => promise<result<response<array<int>>, readError>>
+
+/**
+Reads the response body as `FormData.t`.
+
+Returns the body with the original response, or a `readError` when the native reader rejects.
+*/
+let formData: Response.t => promise<result<response<FormData.t>, readError>>
+
+/**
+Parses the response body as `JSON.t`.
+
+Malformed JSON and other native reader rejections become `readError` values. This function does not
+validate the parsed JSON against an application domain type.
+*/
+let json: Response.t => promise<result<response<JSON.t>, readError>>
+
+/**
+Reads the response body as a string.
+
+Returns the body with the original response, or a `readError` when the native reader rejects.
+*/
+let text: Response.t => promise<result<response<string>, readError>>

src/fetch/SafeFetchRaw.res

@@ -0,0 +1,4 @@
+let fromUrl = (url: string, ~init: option<Request.requestInit>=?) => Fetch.fetch(url, ~init?)
+
+let fromRequest = (request: Request.t, ~init: option<Request.requestInit>=?) =>
+  Fetch.fetchWithRequest(request, ~init?)