[Typing-sig] Re: Result[T]

June 22, 2021

      Rust `Result<T, E>` is an algebraic data type. Python doesn't have equivalent builtin construct, but it can simply be emulated using union and dataclasses. For example:

```python
from typing import TypeVar
from dataclasses import dataclass

@dataclass
class Ok(Generic[T]):
    value: T

@dataclass
class Err(Generic[E]):
    error: E

Result = Ok[T] | Err[E]
```

It does play well with pattern matching:

```python
def unwrap(result: Result[T, E]) -> T:
    match result:
        case Ok(v):
            return v
        case Err(e):
            raise e if isinstance(e, Exception) else Exception(e)

def ok(result: Result[T, E]) -> T | None:
    match result:
        case Ok(v):
            if v is None:
                warnings.warn("ok was called with a None result", RuntimeWarning)
            return v
        case _:
            return None

… # all Result methods can be implemented like that
```

[Typing-sig] Re: Result[T]

Joseph Perez