```
def fill_template(template: str, values: dict) -> str:
    if not isinstance(template, str):
        raise ValueError("fill_template expects a string template")
    # TODO: walk the template once, passing literal characters through
    # and replacing each placeholder from values; reject an unterminated
    # placeholder, an empty or malformed name, and an unknown name.
    return template.replace("{", "").replace("}", "")
```