Metro bundler inlines (ReactNative.)Platform.OS and (ReactNative.)Platform.select, so that when packaging for one platform, the code for the other platform is optimized away.

So if I had

[@bs.module "react-native"] [@bs.scope "Platform"] external os : string = "OS";

if (os === "ios") {
  Js.log("ios");
} else {
  Js.log("android");
};

this would get compiled by BuckleScript to

if (ReactNative.Platform.OS === "ios") {
  console.log("ios");
} else {
  console.log("android");
}

and Metro Bundler would inline ReactNative.Platform.OS and subsequently optimize the Android branch away for an iOS build.

However, with the current Rebolt bindings, I have

switch (Platform.os()) {
| IOS(_) => Js.log("ios")
| Android => Js.log("android")
};

which gets compiled by BuckleScript to

var match = Platform$Rebolt.os(/* () */0);
if (match) {
  console.log("ios");
  return /* () */0;
} else {
  console.log("android");
  return /* () */0;
}

This prevents the inlining/optimization from working.