[Haskell-cafe] Object Oriented programming for Functional Programmers
miguelimo38 at yandex.ru
Wed Jan 2 10:41:54 CET 2013
On Jan 2, 2013, at 2:26 AM, Bob Hutchison <hutch-lists at recursive.ca> wrote:
> On 2013-01-01, at 3:47 PM, MigMit <miguelimo38 at yandex.ru> wrote:
>> Well, probably one of the reasons is that I've learned Eiffel later than Haskell.
>> But really, "Design by Contract" — a theory? It certainly is a useful approach, but it doesn't seem to be a theory, not until we can actually prove something about it, and Eiffel doesn't seem to offer anything in this direction.
> Don't confuse OOSC2 and Eiffel. Eiffel implements the ideas in OOSC2 as best as Meyer can, but they are not the same thing.
Well, we were talking about Eiffel. OOSC2 deserves a few unkind words as well, but I won't go there.
> And, personally, I think I would be willing to call DbC a theory, or a close precursor to a theory.
I don't know about DbC in general, but it's implementation in Eiffel seems to be nothing more than a few ASSERT macros, for some weird reason embedded into the language.
> So, I think, you're saying take away the contracts and the outcome of compilation won't be any different. Whereas take away the types and Haskell is stopped cold. And that difference makes contracts a 'hack' but types not a 'hack'?
I wasn't clear enough, sorry. I'm sure it's due to sleep deprivation. Or coffee deprivation.
See, there are two parts of Eiffel, as I see it. First one is the "contracts" part. Second is… well, everything else. Second part seems to be doing all the "real" job, while the first one is doing something invisible, something that leaves no trace in the final result. Which doesn't mean it's unimportant, of course. The contracts part is designed to help the other part do it's job, but not to do the job by itself. Now, there are two problems with that:
1) The "real job" part needs helping. And a lot of it, actually, one doesn't need to look very closely to see that Eiffel type system is extremely unsafe (for the statically type language).
2) The "contracts" part does a very poor job. Instead of really improving the inherent unsafety, it resorts to testing. And...
2') ...not even the real, thorough testing — contracts system would be quite happy if the program works on the developer's machine. Which is the "works for me" approach certain languages gets rightfully blamed for.
> Seems to me you're ignoring everything that happens between an empty directory and a working program. Contracts help in that process (I say but can't prove).
I agree. They do help — but there are lots of things that help in this transition. Versioning systems. Collaboration tools. Bug tracking software. Text editors. Even debuggers.
> Pre and post conditions with class invariants are neither types nor unit test, something in between. With the wonderful properties of 'useful' and 'executable'.
> Sometimes you just have to settle for the hacks.
>> On Jan 1, 2013, at 11:41 PM, Mike Meyer <mwm at mired.org> wrote:
>>> MigMit <miguelimo38 at yandex.ru> wrote:
>>>> On Jan 1, 2013, at 10:23 PM, Никитин Лев <leon.v.nikitin at pravmail.ru>
>>>>> Eiffel, for my opinion, is a best OOP language. Meyer use a
>>>> theoretical approach as it is possible in OOP.
>>>> Really? Because when I studied it I had a very different impression:
>>>> that behind this language there was no theory at all. And it's only
>>>> feature I remember that is not present in mainstream languages is it's
>>>> pre/postconditions system, which looked like an ugly hack for me.
>>> I agree with Leon. Of course, I learned it out of OOSC2, which provides the theory. When compared to "mainstream" OO languages like C++, Java or Python, it's on a much solider theoretical basis. Compared to something like Scheme, Haskell or even Clojure, maybe not so much.
>>> On the other hand, one persons theory is another persons hack. The theory behind the pre/post conditions is "Design by Contract". The contracts are as important as the type signature, and show up in the auto-generated docs in eiffel systems. I found at least one attempt to add DbC features to Haskell. I'm not sold on it as a programming technique - the bugs it uncovers are as likely to be in the pre/post conditions as in the code.
>>> Sent from my Android tablet with K-9 Mail. Please excuse my swyping.
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