fetch.ts

export type FetchSuccess = { response?: Response } & { responseObject?: JsonObject };

export type FetchError = FetchSuccess & {
  errors: ReadonlyArray<object | string>;
}

export const post = (
  url: string,
  body: object,
) => {
  const doFetch = TE.tryCatch(
    async () => ({
      response: await fetch(url, {
        method: 'POST',
        body: JSON.stringify(body),
      }),
    }),
    error => ({
      errors: [error as object],
    }),
  );

  const parse = (
    result: FetchSuccess, // error is impossible here
  ): TE.TaskEither<FetchError, FetchSuccess> =>
    ((response) => TE.tryCatch(
      async () => ({
        response,
        responseObject: await response.json(),
      }),
      error => ({
        response,
        errors: [error as object],
      }),
    ))(result.response as Response);

  const checkOK = (result: FetchError | FetchSuccess) =>
    ((response) => pipe(response.ok, fold(
      () => ({
        ...result,
        errors: RA.cons(
          `Got ${response.status} POSTing to ${url}`,
          'errors' in result ? result.errors : [],
        ),
      }),
      () => result,
    )))(result.response as Response);

  return pipe(
    doFetch,
    TE.chain(parse),
    TE.map(checkOK),
  );
};