Servant Haskell library issue with Optional ReqBody demonstration

issue-demo.hs

#! /usr/bin/env nix-shell
#! nix-shell --pure -i runhaskell -E "import ./shell.nix {}"

{-
  This a demonstration of the issue with Servant library.
  ReqBody' '[Optional] doesn’t make the value be optional,
  the value of ReqBody isn’t wrapped into Maybe as it supposed to.

  Run server (make sure there’s “shell.nix” in the same directory):
    ./issue-demo.hs

  Both requests should work:
    curl --fail --verbose http://127.0.0.1:8088/test -XPUT
    curl --fail --verbose http://127.0.0.1:8088/test -XPUT \
      -H 'Content-Type: application/json' --data '[1,2]'
-}

{-# LANGUAGE UnicodeSyntax, DataKinds, TypeOperators, TypeApplications #-}
{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, OverloadedStrings #-}

import Data.Proxy
import Control.Monad.IO.Class (liftIO)
import Servant
import Servant.Server
import Network.Wai.Handler.Warp
import Network.HTTP.Media ((//))

data Nope = Nope

type TestApi =
  "test"
  :> ReqBody' '[Optional, Strict] '[Nope, JSON] [Integer]
  :> Put '[JSON] ()

instance Accept Nope where
  -- Servant defaults to application/octet-stream when Content-Type header isn’t
  -- provided
  contentType Proxy = "application" // "octet-stream"

-- A hack in order to default the value into mempty to make it be optional
instance Monoid m ⇒ MimeUnrender Nope m where
  mimeUnrender Proxy _ = Right mempty

server ∷ Server TestApi
server = testHandler where
  -- As you can see here the ReqBody type [Integer] wasn’t wrapped into Maybe
  -- even though it is Optional.
  testHandler ∷ [Integer] → Handler ()
  testHandler = liftIO . print

app ∷ Application
app = serve (Proxy @TestApi) server

main ∷ IO ()
main = putStrLn "Running server..." >> run 8088 app

shell.nix

{ pkgs ?
    let
      src = fetchTarball {
        url =
          "https://github.com/NixOS/nixpkgs/archive/" +
          "a9226f2b3a52fcbbc5587d2fa030729e714f40fe.tar.gz";
        sha256 = "0xlzkymfrkj7z7b6hwliq2zn6pbjw08zka0qyv5bbnkhnv16x1dh";
      };

      # In nixpkgs latest default version of Servant is 0.16.2.
      # Overriding here version to 0.18.
      servantOverlay = self: super: {
        haskellPackages = super.haskellPackages.override {
          overrides = hsSelf: hsSuper: {
            servant        = hsSuper.servant_0_18;
            servant-server = hsSuper.servant-server_0_18;
          };
        };
      };
    in
      import src { overlays = [ servantOverlay ]; }
}:
let
  haskellDeps = p: [ p.servant p.servant-server p.warp p.http-media ];
  ghc = pkgs.haskellPackages.ghcWithPackages haskellDeps;
  shell = pkgs.mkShell { buildInputs = [ ghc ]; };

  info = {
    "GHC version"     = ghc.version;                          # 8.8.4
    "Servant version" = pkgs.haskellPackages.servant.version; # 0.18
  };

  renderedInfo =
    builtins.concatStringsSep "\n" (
      builtins.map (k: "${k} = ${info.${k}}") (builtins.attrNames info)
    );
in
builtins.trace ("\n${renderedInfo}") shell