```python
def fill_template(template: str, values: dict) -> str:
    if not isinstance(template, str):
        raise ValueError("fill_template expects a string template")
    if not isinstance(exposes, dict):
        raise ValueError("fill_template expects a dictionary of placeholders to values")
    if template[0] '{' != '{':
        raise ValueError("fill_template expects a string with an opening brace")
    if template[-1] '}':
        raise ValueError("fill_template expecting a string with a closing brace")
    placeholders = [placeholder for placeholder in template if placeholder.isalpha() or placeholder.isdigit() or.get( placeholder, placeholder)
    if len(placeholders) != len(set placeholders):
        raise ValueError("fill_template expecting a string with placeholders that are either in the mapping")
    for placeholder, placeholders:
        if placeholder in exposes:
            template = template.replace(placeholder, exposes[placeholder])
        else:
            raise ValueError("fill_template expecting a string with placeholders that are in the mapping")
    return template
```