How to write a predicate like:
	assert( throws(someStatement, ErrorType) );
?
Meaning a kind of builtin shortcut for:
	try:
	    someStatement;
	    throw new SomeCustomError();
	catch (ErrorType _) {}
that would also have the advantage of beeing usable as assert variant, unifying unittest checks. I constantly need that.

Denis
_________________
vita es estrany
spir.wikidot.com

spir:

> How to write a predicate like:
> 	assert( throws(someStatement, ErrorType) );

Are you using Design By Contract a lot? Contracts need to contain asserts only...

Bye,
bearophile

On Sunday 23 January 2011 00:47:13 bearophile wrote:
> spir:
> > How to write a predicate like:
> > 	assert( throws(someStatement, ErrorType) );
> 
> Are you using Design By Contract a lot? Contracts need to contain asserts only...

Not necessarily. In fact, in general, the tact that has been taken with Phobos is that you use assertions when you're verifying that Phobos is correct, and you use enforce (or an if-statement which throws an exception) when verifying that arguments given by outside code is correct.

Design By Contract itself says _nothing_ about verifying that a contract is being followed by not. You could use DbC and _never_ check it, making it undefined behavior when a contract is violated. After all, DbC says that the caller _will_ give the function arguments which follow the contract, so if you don't _have_ to check anything. Obviously, if you want more robust code, you actually verify that arguments are per the contract, but you don't _have_ to.

In general, I think that an API should throw an exception if bad arguments are given, not use an assertion. The assertion is _useless_ when dealing with a public API once it's in a library (except if the function is templated), since the assertion will already have been compiled out (assuming that you're dealing with a release build, but that's the most likely case). Using assertions to verify that your own code is internally consistent is great, but it's not so great when you have to deal with 3rd parties.

Regardless, from this post and previous posts that you have made, I get the impression that you're mixing up the concept of Design by Contract and a typical implementation of it. All that DbC says is that the arguments that a caller gives a function must meet certain requirements and that if they do, the result of the function will meet certain requirements. Verifying that the contract is kept has nothing to do with DbC. It's just good practice if you want to minimize bugs.

- Jonathan M Davis

On 01/23/2011 09:47 AM, bearophile wrote:
> spir:
>
>> How to write a predicate like:
>> 	assert( throws(someStatement, ErrorType) );
>
> Are you using Design By Contract a lot? Contracts need to contain asserts only...
>
> Bye,
> bearophile

No, not yet. I don't see your point.

Denis
_________________
vita es estrany
spir.wikidot.com

Jonathan M Davis:

>Not necessarily. In fact, in general, the tact that has been taken with Phobos is that you use assertions when you're verifying that Phobos is correct, and you use enforce (or an if-statement which throws an exception) when verifying that arguments given by outside code is correct.<

Phobos is using enforce mostly as a temporary work-around of the fact that the DMD compiler is not able to automatically switch using a debug version of Phobos or a release version of it when you compile your code in notrelase or relasee mode, so currently contract assertions are vanished unless you are using a template.

Maybe some enforce() will be left in Phobos, but currently they 1) slow down code, 2) kill inlining, 2) there's no way to disable them, 4) turn nothrow functions into throw ones even if there is no need for this in their algorithm, so most of those enforces() will need to be killed from Phobos.


>All that DbC says is that the arguments that a caller gives a function must meet certain requirements and that if they do, the result of the function will meet certain requirements. Verifying that the contract is kept has nothing to do with DbC. It's just good practice if you want to minimize bugs.<

I don't believe this :-)

Bye,
bearophile

On Sunday 23 January 2011 09:24:25 bearophile wrote:
> Jonathan M Davis:
> >Not necessarily. In fact, in general, the tact that has been taken with Phobos is that you use assertions when you're verifying that Phobos is correct, and you use enforce (or an if-statement which throws an exception) when verifying that arguments given by outside code is correct.<
> 
> Phobos is using enforce mostly as a temporary work-around of the fact that the DMD compiler is not able to automatically switch using a debug version of Phobos or a release version of it when you compile your code in notrelase or relasee mode, so currently contract assertions are vanished unless you are using a template.
> 
> Maybe some enforce() will be left in Phobos, but currently they 1) slow
> down code, 2) kill inlining, 2) there's no way to disable them, 4) turn
> nothrow functions into throw ones even if there is no need for this in
> their algorithm, so most of those enforces() will need to be killed from
> Phobos.

enforce is _not_ going away. What _has_ been discussed is having a non-lazy version of enforce for cases where we don't need a lazy version (but it still wouldn't be the default - if nothing else because that would change the behavior of code). There has also been some discussion of making the compiler smart enough to see that a particular paramater doesn't need to be lazy and make it non-lazy if that's the case (allowing you to use enforce both lazily and non- lazily).

And there are plenty of functions in Phobos which _should_ be throwing. I actually don't expect that there are all that many functions in Phobos which are throwing which shouldn't be (though I'd have to check). I'd expect that there _are_ plenty however that could be made nothrow which haven't been (though we'd need a conditional nothrow of sorts for templates just like we need for pure if we want that to really work), but there are also plenty which _should_ be throwing. std.datetime throws quite a bit - typically on bad arguments - and it _should_. However, every function in there that can be marked as nothrow is.

> >All that DbC says is that the arguments that a caller gives a function must meet certain requirements and that if they do, the result of the function will meet certain requirements. Verifying that the contract is kept has nothing to do with DbC. It's just good practice if you want to minimize bugs.<
> 
> I don't believe this :-)

Well, believe what you like, but if you argue that DbC _requires_ that assertions be used, you're going to get people arguing with you.

- Jonathan M Davis