Broken ReasonReact component

error.txt

  /src/ui-re/src/PageHeader.re 8:17-58

   6 _   | Select('a);
   7 _
   8 _ let component = ReasonReact.reducerComponent("PageHeader");
   9 _
  10 _ let make = (~orgName, ~productName, ~tabs : list(('a, string)), ~onTab
       Change : ('a => unit), _children) => {

  This expression's type contains type variables that can't be generalized:
  ReasonReact.componentSpec(state('_a), ReasonReact.stateless,
                             ReasonReact.noRetainedProps,
                             ReasonReact.noRetainedProps, action('_a))

  This happens when the type system senses there's a mutation/side-effect,
  in combination with a polymorphic value.
  Using or annotating that value usually solves it. More info:
  https://realworldocaml.org/v1/en/html/imperative-programming-1.html#side-effects-and-weak-polymorphism

ninja: build stopped: subcommand failed.
>>>> Finish compiling(exit: 1)

PageHeader.re

[%bs.raw {|require('./Header.css')|}];

type state('a) = {selected: 'a};

type action('a) =
  | Select('a);

let component = ReasonReact.reducerComponent("PageHeader");

let make = (~orgName, ~productName, ~tabs : list(('a, string)), ~onTabChange : ('a => unit), _children) => {
    ...component,
    initialState: () => {selected: (tabs |> List.hd |> fst)},
    reducer: (action, state) =>
      switch(action) {
      | Select(a) => ReasonReact.UpdateWithSideEffects({selected: a}, self => onTabChange(self.state.selected))
      },
    render: self => {
    <nav className="bx--cloud-header">
      <div className="bx--cloud-header__wrapper">
        <button className="bx--cloud-header__app-menu" type_="button">
          <svg width="20" height="20" viewBox="0 0 20 20">
            <path d="M3 4h14v1H3zm0 6h14v1H3zm0 6h14v1H3z"></path>
          </svg>
        </button>
        <a href="#" className="bx--cloud-header-brand">
          <h4 className="bx--cloud-header-brand__text">
            (ReasonReact.string(orgName ++ " "))
            <span>
              (ReasonReact.string(productName))
            </span>
          </h4>
        </a>
        <ul className="bx--cloud-header-list">
          <li className="bx--cloud-header-list__item">
            (ReasonReact.array(
              Array.of_list(List.map(((a, l)) =>
                                     <a className="bx--cloud-header-list__link"
                                     href="#" key=l
                                     onClick=(_e => self.ReasonReact.send(Select(a)) )>
                          (ReasonReact.string(l))
                          </a>, tabs))))
          </li>
        </ul>
      </div>
    </nav>
  },
};