# std/fallible

`Fallible`: the protocol `?` resolves through for a type that is not `Option` or `Result`.

`x?` returns the whole sum it was given, byte for byte, into a function of the same type. So the protocol answers only which side a value is on and how to read the success payload; any number of failing variants propagate intact.

```vyrn
impl Fallible for Http {
    type Output = Body
    fn isSuccess(self) -> Bool { .. }
    fn success(self) -> Body { .. }   // total: `panic` on the failing arms
}

```

`?` calls `success` only after `isSuccess` answered true.

## Fallible

`protocol`, at line 16.

```vyrn
protocol Fallible {
    type Output
    fn isSuccess(self) -> Bool
    fn success(self) -> Output
}
```

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