When there's a definition like:

parseJSON :: forall a. (ReadForeign a) => String -> Either String a
parseJSON json = fromString json >>= parseForeign read
  where
  fromString :: String -> Either String Foreign
  fromString = runFn3 fromStringImpl Left Right

As fromString doesn't use any variables introduced by parseJSON it can be treated as:

parseJSON :: forall a. (ReadForeign a) => String -> Either String a
parseJSON json = fromString json >>= parseForeign read

fromString :: String -> Either String Foreign
fromString = runFn3 fromStringImpl Left Right

As otherwise, in the generated JS, whenever parseJSON is called you also have to pay for the creation of fromString every time:

var parseJSON = function (__dict_ReadForeign_0) {
    return function (json) {
        var fromString = Data_Function.runFn3(fromStringImpl)(Data_Either.Left.create)(Data_Either.Right.create);
        return Prelude[">>="](Data_Either.bindEither({}))(fromString(json))(parseForeign(read(__dict_ReadForeign_0)));
    };
};

This applies anywhere a let or where introduces a function "later" than it needs to, just need to make sure that when moving fromString up its name doesn't conflict with another declaration in the scope it is moved to.