SignupForm.jsx

const initialValues = {
  username: "",
  email: "",
  password: "",
  passwordConfirmation: "",
}

// always give your component a name, even if it is the default export,
// because it will appear in the Component Trace when there is a React error
export default function SignupForm() {
  const [values, dispatch] = useReducer(formReducer, initialValues)

  // dispatch is guaranteed to be persistent, so you can use it in another hook without
  // needing to add it to the dependency array. this keeps your dependencies simple
  const handleChange = useCallback(e => dispatch(change(e)), [])
  const handleReset = useCallback(() => dispatch(reset()), [])

  // you don't "need" useCallback for this simple case, you can also write:
  //     const handleChange = e => dispatch(change(e))
  // or pass `dispatch` as a prop to a child component.
  
  return (
    <form>
      <label>Username <input name="username" onChange={handleChange} value={values.username} /></label>
      ...
      <button type="button" onClick={handleReset}>Reset</button>
      <button>Submit</button>
    </form>
  )
}

// action creators give you a nice high-level summary of how your state can change
const change = e => ({ type: 'change', payload: { field: e.target.name, value: e.target.value } })
const reset = (payload=initialValues) => ({ type: 'reset', payload })

// finally the reducer contains the nitty gritty details
function formReducer(state, action) {
  switch(action.type) {
    case 'change': {
      const { field, value } = action.payload
      return { ...state, [field]: value }
    }
    case 'reset': {
      return action.payload
    }
    default: {
      throw new Error(`Unrecognized type: ${action.type}`)
    }
  }
}

Had to revise a couple things to adapt this to work.

For the dispatches:
const handleChange = e => { dispatch(change(e)); }; const handleReset = () => { dispatch(reset()); };

And for the inputs:
<label> Email <input type="email" name="email" onChange={e => handleChange(e)} value={values.email} /> </label>

Without the e => handleChange(e) it kept complaining about it not being a function and it being an object.