That’s not invalid state. Your program correctly determined input is invalid.
Say user input is a number and can never exceed 5. If the user input exceeds 5 your program should handle that gracefully. This is not invalid state. It is handling invalid input while remaining in valid state.
Let say it does exceed 5 and You forget to check that it should never exceeds 5 and this leads to a division by zero further down your program. You never intended for this to happen and you don’t k ow why this happened. This is invalid state.
Now either this division by zero leads to an error value or it can throw an exception. It doesn’t matter. Do you know how to recover? You don’t even know where the bug is. A lot of times your don’t even know what to do. This error can bubble up all the way to main and now what do you do with it?
You crash the program. Or you can put in a fail safe before the error bubbles up to main and do something else but now (if your program retains and mutates that state) has invalid values in it and a known bug as well.
Imagine that input number represented enumeration values for movement of some robot. You now have one movement that doesn’t exist or was supposed to be something else. Thus if you keep your program running the robot ends up in an unexpected and erroneous place. That’s why I’m saying a program should crash. It should not be allowed to continue running with invalid state.
Like imagine if this was a mission critical auto pilot and you detect negative 45 meters for altitude. Bro crash and restart. Don’t try to keep that program running by doing something crazy in attempt to make the altitude positive and correct. Reset it and hope it never goes to the invalid state again.
> Or you can put in a fail safe before the error bubbles up to main and do something else
In other words,
>> your parser backtracked from parsing a Bool and decided to try to parse an Int instead
> but now (if your program retains and mutates that state) has invalid values in it and a known bug as well.
Unless,
>>> The thing with functional programming (specifically, immutable data,) is that as long as the invalid state is immutable, you can just back up to some previous caller, and they can figure out whether to deal with it or whether to reject up the its previous caller.
Your parser backtracking from a bool when it tried to parse an int is VALID behavior. Your parser tries to parse text into several possible types at runtime. Backtracking is STILL valid state. You’re not thinking clearly.
Look at my auto pilot example. You have a bug in your program you don’t know about. Altitude reads -9999 meters your heading and direction is reading at the speed of light. Your program sees these values and recognizes invalid state. There is a BUG in your program. Your program WAS never designed to go here.
You want to try to recover your autopilot program? How the fuck are you gonna do that? You don’t even know where the bug is. Does your recovery routine involve patching and debugging? Or are you gonna let your autopilot keep operating the plane with those nonsense values? That autopilot might hit the wind breaks to try to slow down the plane and end up crashing the thing.
You don’t recover this. You restart the program and pray it doesn’t hit that state again then when you are on the ground you debug it.
>>> The thing with functional programming (specifically, immutable data,) is that as long as the invalid state is immutable, you can just back up to some previous caller, and they can figure out whether to deal with it or whether to reject up the its previous caller.
This makes no fucking sense. Where the hell will you back up to? Your error came from somewhere but you don’t know where. Return an error value in the child function the parent function receives the error and returns an error itself and this keeps going until you bubble up to the main function because you don’t know where that error came from.
Now you have an error in the main function. Wtf are you gonna do? How do you handle an error in the main function that you have no idea where it came from? Here’s an idea. You have the main function restart the program loop. See a similarity here? It’s called crashing and restarting the program. Same effect!
This isn’t a functional programming versus non functional thing. It’s a concept that you’re not seeing.
I know this is not your point for the last paragraph, but have you read about the C-130 complete navigation system failure while trying land below sea level at the Dead Sea? :)
Say user input is a number and can never exceed 5. If the user input exceeds 5 your program should handle that gracefully. This is not invalid state. It is handling invalid input while remaining in valid state.
Let say it does exceed 5 and You forget to check that it should never exceeds 5 and this leads to a division by zero further down your program. You never intended for this to happen and you don’t k ow why this happened. This is invalid state.
Now either this division by zero leads to an error value or it can throw an exception. It doesn’t matter. Do you know how to recover? You don’t even know where the bug is. A lot of times your don’t even know what to do. This error can bubble up all the way to main and now what do you do with it?
You crash the program. Or you can put in a fail safe before the error bubbles up to main and do something else but now (if your program retains and mutates that state) has invalid values in it and a known bug as well.
Imagine that input number represented enumeration values for movement of some robot. You now have one movement that doesn’t exist or was supposed to be something else. Thus if you keep your program running the robot ends up in an unexpected and erroneous place. That’s why I’m saying a program should crash. It should not be allowed to continue running with invalid state.
Like imagine if this was a mission critical auto pilot and you detect negative 45 meters for altitude. Bro crash and restart. Don’t try to keep that program running by doing something crazy in attempt to make the altitude positive and correct. Reset it and hope it never goes to the invalid state again.