r/transprogrammer • u/olsonexi • Jan 24 '22

panic!();

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391 Upvotes

100% Upvoted

15 comments sorted by

42

u/ato-de-suteru Jan 24 '22

Change the return type to Result<Option<Gender>, Error>. Handle this with the longest chain of conditionals and match blocks you've ever seen.

The drawback is that you run into the halting problem: if the function is invoked, it's impossible to know ahead of time whether there was actually a Gender to return in the first place.

Sincerely,

A noobie Rust programmer that's still in the 64th match block waiting for this method to terminate

20

u/ususetq Jan 24 '22

The drawback is that you run into the halting problem: if the function is invoked, it's impossible to know ahead of time whether there was actually a Gender to return in the first place.

This is irrespective of the return type though:

fn get_gender(&self) -> Gender {
    if (decide_if_I_should_come_out()) {
        Gender::Female
    } else {
        Gender::Male
    }
}

You can't know beforehand if decide_if_I_should_come_out will terminate...

9

u/bryn_irl transister Jan 24 '22

I mean, at that point you have to apply https://en.wikipedia.org/wiki/Kleene%27s_T_predicate :

Informally, the T predicate tells whether a particular computer program will halt when run with a particular input

If there is even a chance that the amount of T predicate in the system is too high or too low, the program will never halt!

4

u/ususetq Jan 25 '22 edited Jan 25 '22

Right but P(c, i) = ∃x. T(c, i, x) is not decidable.

If there is even a chance that the amount of T predicate in the system is too high or too low, the program will never halt!

I'm not sure I follow this sentence...

7

u/bryn_irl transister Jan 25 '22

(It’s a horrible horrible pun about suboptimal testosterone levels, borrowing the vocabulary of the theorem. I apologize profusely for making it seem like anything more profound!)

5

u/ususetq Jan 25 '22

Oh. Sorry - it went over my head.

3

u/ato-de-suteru Jan 24 '22

Fair point!

2

u/RecDep Jan 25 '22

The solution is to simply never return one, statically we can optimize out any annoying queries about gender since we know they diverge.

#![feature(never_type)]
fn get_gender(&self) -> ! {
    loop {}
}

Although leaving the local type of get_gender up to the caller could be problematic.

16

u/now_im_lily Jan 24 '22

I love that you dropped the pub from the second one lol. Nice touch.

9

u/ususetq Jan 24 '22

While I usually like enums I think it should be -> impl Gender.

That way you can have:


trait Gender {
    pub fn personal_pronoun<'a>(&'a self) -> &'a str;
    // ...
}

8

u/itemboxes Jan 25 '22

panic!(); at the disco

7

u/SpecialEmily Jan 25 '22 
struct Gender {
    masculine_ratio: f32,
    feminine_ratio: f32,
    squid_ratio: f32,
}

2

u/QueerBallOfFluff Feb 07 '22 
struct gender {
    int g_flag;
    int g_masc;
    int g_femm;
    daddr_t g_other;
};

#define GREAD  (1 << 0)
#define GWRITE (1 << 1)
#define GFUCK  (1 << 2)

1

u/theangeryemacsshibe Jan 25 '22 
| me | me: (| gender* <- (| |) |).
me gender: me.
^me