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turing completeturing completePosted Dec 20, 2011 0:57 UTC (Tue) by HelloWorld (guest, #56129)In reply to: turing complete by tialaramex Parent article: Cracks in the Foundation (PHP Advent)
Accusing people of "trolling" when they explain why you're wrong is poor form.I'm not. The general idea you had was fine, there are indeed applications which necessarily involve side effects and the lambda calculus doesn't have side effects. But answering individual HTTP requests (remember PHP isn't a web server, it doesn't need to care about the details of how those requests are made, where they come from, or how answers are returned) isn't really one of those applications. Thus the lack of side effects isn't a deal breaker, and my explanation deliberately didn't use them.Yeah, except that it *is* a deal breaker because you need a lot more than just the contents of the HTTP request in order to do anything meaningful (i. e. something that couldn't be done just as well with JavaScript on the client side). You know, things like access the database or communicate with other hosts. OK, you can get the database thing to work by passing the entire contents of the database to your lambda expression and have it return a tuple containing the response and the complete new database contents. Not only may one consider that "cheating" (as you need a more and more elaborate machinery outside of the lambda calculus expression, to the point that you can't actually claim to have done anything meaningful in said expression any longer), but that approach also quickly leads to a dead end where you'd need to pass the state of the whole world to the lambda expression in order to obtain the desired answer. And actually, even that wouldn't suffice. Imagine a web server with a hardware random number generator and a web application that allows you to query the xth number generated from now on (where x is supplied by the client). Due to Heisenberg's uncertainty principle, you can't possibly measure the state of the RNG well enough to predict what the xth generated number will be, so you clearly need some kind of side effect for that kind of application. The reason I didn't write all this stuff earlier is that I just couldn't be bothered to argue with a troll like nybble41. There's simply no value at all in this discussion, and nothing enlightening is going to come out of it (not for me, anyway). The only reason I engaged in it at all is that I don't like being told I'm wrong when I'm not.
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turing complete Posted Dec 20, 2011 4:23 UTC (Tue) by tialaramex (subscriber, #21167) [Link]
So your main complaint now is that the untyped lambda calculus can't pretend to be a hardware random number generator? Are you even paying attention to the discussion? Hardware random number generators aren't a PHP feature. The question wasn't "can a lambda expression correctly simulate the hardware of an entire web server?".
Your objection to state cookies is spurious. The state cookie is opaque outside of the lambda expression, all the hard work happens _inside_ the lambda expression. If the state held between requests is small and client-specific you can sidestep this altogether by asking the user agent to hold on to it for you, in your HTTP response. In fact you may recall that recently Microsoft ASP.NET was found to have introduced a serious security vulnerability by transmitting inadequately encrypted session state in user-accessible HTTP cookies.
It's certainly not fair to pretend that most, or even many web sites depend somehow upon the "state of the whole world". Usually there's a modest database, and perhaps some files stored on disk. A pure functional system could easily wrap all that material into the state cookie. Many sites _appear_ to integrate content from several other systems, but actually that's usually done client side in the user agent these days, the "integration" from the point of view of PHP or our hypothetical lambda expression is just a few lines of unchanging plain text returned in the HTTP response.
There are numerous _practical_ problems which keep web sites from being built out of say, pure Lisp, but theoretically there's no real obstacle.
turing complete Posted Dec 20, 2011 9:50 UTC (Tue) by HelloWorld (guest, #56129) [Link]
> So your main complaint now is that the untyped lambda calculus can't pretend to be a hardware random number generator?
No, it's not. But your response shows yet again that this discussion is utterly pointless. You don't want to get the point, fine with me. Have a nice life.
turing complete Posted Dec 20, 2011 10:39 UTC (Tue) by smurf (subscriber, #17840) [Link]
Please engage brain.
A PHP without external state is as powerful as a pure Lambda calculus. That's a fact.
PHP has features to talk to the external worls, save state, and whatnot, because it's designed to be (mostly) useful. A pure lambda calculus has not, because it's designed to prove computability theorems and related stuff.
From a mathematical PoV, that's rather superficial. You can write a Web server in COBOL or Intercal or even Brainfuck if you add the required features somehow, and you can write a Prolog theorem prover in them too.
Doesn't mean that doing any of this makes any practical sense whatsoever.
turing complete Posted Dec 22, 2011 3:18 UTC (Thu) by bronson (subscriber, #4806) [Link]
If you're going to concede, why not concede gracefully?
turing complete Posted Dec 22, 2011 5:32 UTC (Thu) by raven667 (subscriber, #5198) [Link]
Where's the fun in that? Go out guns blazing.
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