fetch.ts

import * as TE from "fp-ts/lib/TaskEither"
import { pipe } from "fp-ts/lib/pipeable"
import { Do } from "fp-ts-contrib/lib/Do"

type Error1 = { errors: object[]; type: "1" }
type Error2 = { errors: object[]; response: Response; type: "2" }
type Error3 = { errors: object[]; result: unknown; type: "3" }
type Errors = Error1 | Error2 | Error3

export const post = (url: string, body: object) =>
  Do(TE.taskEither)
    .bind(
      "response",
      TE.tryCatch(
        () =>
          fetch(url, {
            method: "POST",
            body: JSON.stringify(body),
          }),
        (error) =>
          ({
            errors: [error as object],
            type: "1" as const,
          } as Error1),
      ),
    )
    .bindL("result", ({ response }) =>
      pipe(
        TE.tryCatch(
          () => response.json(),
          (error) =>
            ({
              response,
              errors: [error as object],
              type: "2",
            } as Error2),
        ),
        TE.mapLeft((err) => err as Errors),
        TE.chain((result) => {
          if (!response.ok) {
            return TE.left({
              result,
              response,
              errors: RA.cons(
                `Got ${response.status} POSTing to ${url}`,
                "errors" in result ? result.errors : [],
              ),
            })
          }
          return TE.right(result)
        }),
      ),
    )
    .return(({ result }) => result)

Disclaimer: I do not recommend to write a fetch function like this ;-)