Because you're trying to manually optimize code, which is kindof like saying 'I konw what I'm doing so TypeScript please shut up'. Take for example the `map_err` function of the `Ok` class. The function returns a different `Result`, but you're using the `… as any` trick to shut up the compiler. The correct implementation would be:

    map_err<U>(fn: (arg: E) => U): Result<T, U> {
        return new Ok<T, U>(this.value)
    }

I see why you're not doing that, because that allocates a new object, and that might seem bad. Even the implementation in Rust 'allocates' a new object, but then it relies on the rust compiler to optimize that out. Here's the relevant code:

    pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
        match self {
            Ok(t) => Ok(t),
            Err(e) => Err(op(e))
        }
    }

See the `Ok(t) => Ok(t)` line? That's the same as `return new Ok(…)` in the TypeScript code. Unfortunately TypeScript is not an optimizing compiler, so it's less powerful than rust. You can either use tricks (`… as any`) and give up type safety or be truthful at the expense of performance.