Partial from reducer field

app.re

[@bs.val] [@bs.scope "performance"] external now : unit => float = "";

...

| TouchStart(clientX) =>
  if (state.nav.isOpen) {
    state.nav.isSwiping := true;
  };
  ReasonReact.Update({
    ...state,
    nav: {
      ...state.nav,
      position: [(clientX, now())]
    }
  });
| TouchMove(clientX) =>
  if (state.nav.isSwiping^) {
    ReasonReact.Update({
      ...state,
      nav: {
        ...state.nav,
        position: [(clientX, now()), ...state.nav.position]
      }
    });
  } else {
    ReasonReact.NoUpdate;
  }
| TouchEnd(clientX) =>
  state.nav.isSwiping := false;
  let velocity =
    switch state.nav.position {
    | [] => 0.0
    | [_] => 0.0
    | [(x', t'), (x, t), ..._] => (x' -. x) /. (t' -. t)
    };
  if (velocity < (-0.3)) {
    ReasonReact.Update({
      ...state,
      nav: {
        ...state.nav,
        isOpen: false
      }
    });
  } else if (clientX < 300.0 /. 2.0) {
    ReasonReact.Update({
      ...state,
      nav: {
        ...state.nav,
        isOpen: false
      }
    });
  } else {
    ReasonReact.Update(state);
  };