A final switch on an enum complains if you don't handle all the enum's cases. I like this feature.

However, sometimes the data I'm switching on is coming from elsewhere (i.e. a user), and while I want to enforce that the data is valid (it's one of the enum values), I don't want to crash the program if the incoming value is not correct. But final switch doesn't let me declare a default case (to throw an exception instead).

If I use a non-final switch, then my code might forget to handle one of the cases.

Oh, and to throw a monkey wrench in here, the value is a string, not an integer. So I can't use std.conv.to to verify the enum is valid (plus, then I'm running a switch twice).

Any ideas on better ways to handle this?

-Steve

On Thu, Jun 17, 2021 at 05:41:28PM -0400, Steven Schveighoffer via Digitalmars-d-learn wrote: [.[..]
> Oh, and to throw a monkey wrench in here, the value is a string, not an integer. So I can't use std.conv.to to verify the enum is valid (plus, then I'm running a switch twice).
> 
> Any ideas on better ways to handle this?
[...]

Why not just:

	try {
		MyEnum value = input.to!MyEnum;
	} catch (Exception e) {
		stderr.writeln("Invalid input");
	}

?


T

-- 
People demand freedom of speech to make up for the freedom of thought which they avoid. -- Soren Aabye Kierkegaard (1813-1855)

On Thursday, 17 June 2021 at 21:41:28 UTC, Steven Schveighoffer wrote:
>
> However, sometimes the data I'm switching on is coming from elsewhere (i.e. a user), and while I want to enforce that the data is valid (it's one of the enum values), I don't want to crash the program if the incoming value is not correct. But final switch doesn't let me declare a default case (to throw an exception instead).
>
> If I use a non-final switch, then my code might forget to handle one of the cases.
>
> Any ideas on better ways to handle this?

Perhaps just a non-final switch, with a static assert comparing the number of cases handled to the number of enum members? (managable if the number of cases is small and easily countable)

-Johan

On 6/17/21 5:54 PM, H. S. Teoh wrote:
> On Thu, Jun 17, 2021 at 05:41:28PM -0400, Steven Schveighoffer via Digitalmars-d-learn wrote:
> [.[..]
>> Oh, and to throw a monkey wrench in here, the value is a string, not
>> an integer. So I can't use std.conv.to to verify the enum is valid
>> (plus, then I'm running a switch twice).
>>
>> Any ideas on better ways to handle this?
> [...]
> 
> Why not just:
> 
> 	try {
> 		MyEnum value = input.to!MyEnum;
> 	} catch (Exception e) {
> 		stderr.writeln("Invalid input");
> 	}
> 

Yeah, that was a possibility I listed. Of course `to` is going to do a switch, which means then after I validate the value is valid, I then have to do a final switch to process it. I'd prefer to do it both in one switch.

-Steve

On 6/18/21 6:35 AM, Johan wrote:
> On Thursday, 17 June 2021 at 21:41:28 UTC, Steven Schveighoffer wrote:
>>
>> However, sometimes the data I'm switching on is coming from elsewhere (i.e. a user), and while I want to enforce that the data is valid (it's one of the enum values), I don't want to crash the program if the incoming value is not correct. But final switch doesn't let me declare a default case (to throw an exception instead).
>>
>> If I use a non-final switch, then my code might forget to handle one of the cases.
>>
>> Any ideas on better ways to handle this?
> 
> Perhaps just a non-final switch, with a static assert comparing the number of cases handled to the number of enum members? (managable if the number of cases is small and easily countable)

Hm... interesting idea! I don't know how to count the number of cases, but possibly a CTFE validation would be possible (basically use CTFE to try all the switch cases, and if any throws an exception, then you found one that's not handled).

-Steve

On 6/18/21 12:40 AM, Mathias LANG wrote:
> On Thursday, 17 June 2021 at 21:41:28 UTC, Steven Schveighoffer wrote:
>> A final switch on an enum complains if you don't handle all the enum's cases. I like this feature.
>>
>> However, sometimes the data I'm switching on is coming from elsewhere (i.e. a user), and while I want to enforce that the data is valid (it's one of the enum values), I don't want to crash the program if the incoming value is not correct. But final switch doesn't let me declare a default case (to throw an exception instead).
>>
>> If I use a non-final switch, then my code might forget to handle one of the cases.
>>
>> Oh, and to throw a monkey wrench in here, the value is a string, not an integer. So I can't use std.conv.to to verify the enum is valid (plus, then I'm running a switch twice).
>>
>> Any ideas on better ways to handle this?
>>
> 
> Well, if you receive an `enum` that have an out of bounds value, your problem lies in the caller, not the callee. You're breaking the most fundamental promise of a type, that is, the values it can take. And you obviously also break any `@safe` function by feeding it this value.

Yeah, I know. But I'm not receiving an enum. I'm receiving a string. But I want to handle a certain set of those strings everywhere. So what I tried is to make an enum that has those strings. Then I would use final switches whenever I handle it, so if I add a new string to the list, the compiler will tell me where I missed handling that new one.

> 
> So instead of thinking in terms of `enum`, I would say, think in them of the value, and generate the switch:
> ```D
> SWITCH: switch (myRawValue)
> {
>      static foreach (EV; NoDuplicates!(EnumMembers!MyEnum))
>      {
>          case EV:
>              // Handle;
>              break SWITCH;
>      }
>      default:
>          throw new Exception("Invalid value: " ~ myRawValue);
> }
> ```
> 

The // Handle then becomes a new switch. Though maybe I can group some of them together.

I may as well use std.conv.to at that point.

I think that's what I'm probably going to do, I just wondered if there was a better way.

-Steve