Parameterize a protocol

A protocol can describe the same operation for several result types. Classes satisfy it by providing matching methods.

protocol Readable[T]:
    def read(self) -> T:
        pass

class Cell[T]:
    value: T
    def read(self) -> T:
        self.value

def read[T, R: Readable[T]](source: &R) -> T:
    source.read()

def main() -> Result[(), IoError]:
    cell = Cell(7u8)
    print(read(&cell))?
    Ok(())

7

The argument determines R = Cell[u8]; that class’s read signature determines T = u8. Its return type and receiver borrowing must match the protocol. Explicit read[u8, Cell[u8]](&cell) also works. Conflicting type evidence is an error, rather than a reason to widen integers.