I think Associative Array should throw Exception

Paul Backus snarwin at gmail.com
Fri Sep 4 13:43:40 UTC 2020

On Tuesday, 1 September 2020 at 18:20:17 UTC, Jesse Phillips 
> This is going to be a hard one for me to argue but I'm going to 
> give it a try.
> Today if you attempt to access a key from an associative array 
> (AA) that does not exist inside the array, a RangeError is 
> thrown. This is similar to when an array is accessed outside 
> the bounds.
> [...]
> I don't have an issue with the normal array RangeError, there 
> is a clear means for claiming your access is a programming bug. 
> However associative arrays tend to have both the key and value 
> as "input."
> [...]
> Is it recoverable? I would say yes. We aren't actually trying 
> to access memory outside the application ownership, we haven't 
> put the system state into a critical situation (out of memory). 
> And a higher portion of the code could easily decide to take a 
> different path due to the failure of its call.

Any time you have an operation that can only succeed if some 
precondition is met, there are two possible ways you can 
implement it:

1. Make it the caller's responsibility to check the precondition.
2. Make it the function's responsibility to check the 

If you have version #1, you can always use it to implement 
version #2, but the converse is not true. So, while you would 
ideally provide both versions and let the user choose the one 
they prefer, you should always *at least* provide version #1.

In this case, for D's associative arrays, the [] operator is 
version #1. You could make a reasonable case that the [] operator 
should have been reserved for version #2, and version #1 should 
have been named something else, but at this point, it's not worth 
breaking backwards compatibility to change it.

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