partial from reducer field

app.re

| TouchStart(clientX) =>
  if (state.nav.isOpen) {
    state.nav.isSwiping := true;
  };
  ReasonReact.Update({
    ...state,
    nav: {
      ...state.nav,
      position: [clientX]
    }
  });
| TouchMove(clientX) =>
  if (state.nav.isSwiping^) {
    ReasonReact.Update({
      ...state,
      nav: {
        ...state.nav,
        position: [clientX, ...state.nav.position]
      }
    });
  } else {
    ReasonReact.NoUpdate;
  }
| TouchEnd(clientX) =>
  state.nav.isSwiping := false;