Transform a cell with a generic method

Give a method its own type parameters to let a callback transform Cell[T] into Cell[U]:

class Cell[T]:
    value: T

    def map[U, F: Callable[[T], U]](self, transform: &F) -> Cell[U]:
        Cell(transform(self.value))

def double(value: u8) -> u16:
    # u8 is at most 255, so the doubled value is at most 510 in u16.
    unchecked:
        u16(value) * 2

def main() -> Result[(), IoError]:
    original = Cell(7u8)
    transform = double
    changed = original.map(&transform)
    print(changed.value)?
    print(original.value)?
    Ok(())

14
7

map borrows the original cell, so passing its field to the callback requires a copyable T. Here, double determines the result type U = u16.