Re: Discriminants or Constructor Function for Limited Types

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2022-05-09 12:19, Doctor Who wrote:

> if you do a check before write there is no need to rise and propagate
> an exception.

A bad idea.

First it is an unnecessary overhead, because ultimately the check will 
be repeated.

Secondly it is technically impossible to do for a huge number of reasons:

1. Too complex to do. The modern hardware and software does a lot of 
bookkeeping, indexing, replicating, compression, encryption, the stuff 
almost impossible to estimate or predict in advance.

2. Unreliable due to racing conditions, volatile network states etc.

3. Very expensive, e.g. in the case of a networking file system or a 
database and the asynchronous nature of modern I/O subsystems. When you 
write a file you normally do not wait for the operation completion. 
Actual writing continues asynchronously to your application going 
through dozens of stacks, caches, buffers, buses. If that fails you will 
learn that later in a consequent operation unless you explicitly call 
Flush or equivalent. Checking the state is a synchronous operation that 
would have hundred- to thousandfold performance penalty.

4. Just impossible like when dealing with a stream, a pipe line etc.

As general design rules regarding exceptions consider:

1. Never use return code.

2. Never check anything non-trivial in advance.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de