The Convergence of Modern C++ on the Lisp Programming Style
chriskohlhepp.wordpress.com
chriskohlhepp.wordpress.com
I highly recommend watching Alex Stepanovs A9 lecture series, "Efficient Programming with Components" [0], where he covers the humble min() function, and variants thereof, for at least ~20 hours. During this time he also takes a minor digression in to writing a memory pool for linked-list nodes, while pointing out similarities in the design to a Lisp. It's extremely 'instructive', as he would say, in understanding how he writes C++ code. The sheer amount of code will horrify you, it may even seem unproductive, but I think his objective was really to get everyone thinking about the beauty of the algorithms and the details rather than the objective. His concern for performance, correctness of code, and composition of algorithms is interesting and something I haven't seen presented in real time, in C++, in such a manner before. One example for instance, are occasions where he insists on writing functions backwards, from the return statement up.
If you want a higher level overview on his opinions of C++ and programming etc. The less intense series, "Programming Conversations" [1], is worth a more casual watch.
[0] https://www.youtube.com/playlist?list=PLHxtyCq_WDLXryyw91lah...
[1] https://www.youtube.com/playlist?list=PLHxtyCq_WDLXFAEA-lYoR...
It's also one of the few languages in common use that allows using a wide range of abstraction levels, whatever is desired - I'm not aware of any other language that lets you write inline Asm, procedural, OOP, and functional-style code, all conceivably in the same source file.
Also I'd posit that inline ASM is a lot easier to reason about in C++ than it is in Lisp, given to the rigid type structure. You're bound to shoot yourself in the foot in Lisp, even more than in C++.
I believe it's also a matter of how these languages were designed; Lisp started out as exclusively high-level, while C++ came from C which came from Asm. In other words, the features in C++ almost all have direct mappings to how it could be done in C, and going from C to Asm is also quite direct. In contrast, Lisp -> Asm is a huge conceptual leap.
I think the much-maligned complexity of C++ is because it grew in this step-by-step fashion and exposes all these levels of abstraction so you can easily reason about the tradeoffs involved and understand the code the machine will eventually be running. More importantly, you don't have to use all of them, but this is what makes the language so flexible and accommodating to a wide audience.
(Personally I prefer C/Asm more because I don't operate at such high abstraction levels often, and when I do, the features of C++ are not a good fit for what I want to accomplish. I have used closures in C and done functional-style there, but it doesn't seem like I need to do this often enough to move to C++ or a Lisp.)
Those are merely implementation details of early Lisp leaking to the language naming.
Gambit-C.
At any rate, I was addressing generating and then running Asm from Lisp issue.
If we are to talk about types, I'd hope people would look at languages like Haskell or Standard ML—or maybe even ATS, Agda, or Coq—for future direction, not Lisp.
Unlike Lisp, C++ lets one write efficient, generic algorithms that can operate on several types of data. Lisp cannot, and falls back to dynamic type testing, which makes for slower code[1]. Basically your only option in Lisp is to specialize everything manually, or inline everything. Both approaches are extremely poor. As I hinted, Haskell and Standard ML do an even better job than both C++ or Lisp. This is talked about a good bit in this article [2].
(Also, for the record, Lisp has no concept of information hiding, interfaces, or APIs. And no, Lisp's packages are just convenient structures for grouping somehow related symbols together.)
The article talked about declaring function types in Lisp. Many implementations plainly ignore those and they provide no value. It doesn't help that idiomatic Lisp code usually doesn't include such declarations.
Lastly, a lot of the things in this article were SBCL specific, and had little to do with the Lisp language itself.
Like others have said, I don't think any points demonstrate that C++ is converging to Lisp. And if it were, that's probably a bad direction anyway, given the author's initial statements about heading toward an algorithmic language.
[1] No implementation of Lisp has true, flexible compile-time polymorphism or any type-algebraic structure; all inferred types must be concretely realized during the inference.
Dude was talking about Common Lisp which has (IMO) one of the best object-oriented systems in history, from the I-get-so-happy-when-I-can-use-it perspective.
The arguments you make apply equally to Python, btw.
Also, I'm not a "Haskeller". I do program in Haskell occasionally, though. I'm actually, at this time, a professional Common Lisp programmer.
I also wanted to provide Haskell as a kind of system which provides an inference engine that is much stronger than Lisp's. I don't want anyone to get the idea that Lisp implementations' inference engines is even commensurable to what exists elsewhere.
auto xplusone = [](auto x){ return x + 1; };
auto foo = [](auto n){ return [=](auto i) mutable { return n += i; }; };
Perhaps it's just me, but it seems like an awful lot of conceptual baggage to do things that can be expressed with much greater simplicity and without resorting to concepts that need multiple years of expert knowledge of the language to get this "elegance". And i understand the theoretical elegance, it's just that I have to ask myself if this truly makes an actual difference in code style, simplicity, and clarity of language.
Do you have any suggestions on what tools the people doing 'parlor tricks' with C++11 should be using to accomplish them, then? That is, what would you suggest to get to that place without the straightjacket?
Note that at a bare minimum, these supposed tools should give the ability for fine-grained manual resource control, zero-(runtime)-cost abstractions, and performance roughly on-par with C++. As evidence that these hypothetical tools work well for the purpose, we could look for some complex and high-performance software written in them: say, a browser engine, a 3d engine, a kernel, etc., but wait a minute -- these are all things that tend to be written in C++ or plain C.
Instead of glibly dismissing the language that's used to implement, say, every major browser engine, wouldn't it be more productive to ask questions about why people use it? Bonus points if the answer is something more realistic than "They don't know Lisp".
That way, you end up trying to figure out how to make a replacement for it that is an actual replacement -- Rust is a fantastically exciting example of this.
It'll be great when we can all move away from C++, since it's a colossal clusterfuck of counterproductive complexity, but "C++ is a bad language" misses the point in a really uninteresting kind of way.
Performance? Lisp can give you that, as can OCaml, Haskell, and other better languages. Real time system? There is a mountain of research on real-time garbage collection and on using HLLs for real-time systems. Operating system kernel? OSes were once written in Lisp, and OSes could conceivably be written in other HLLs.
Throw in a requirement to interoperate with a C library and suddenly things get ugly. Yeah, sure, you have an FFI, but debugging across a language barrier is difficult (I have had to do it, it is agony). Suddenly you need to worry about pinning objects so that the garbage collector won't move them while some C library expects them to stay still. In some cases your code basically becomes C but with the syntax of an HLL, and you start to wonder why you did not just write that routine in C to begin with (it would have made your life easier). Performance matters but your compiler needs to set up a trampoline so that your code can provide some kind of callback, and now that is a bottleneck that kills all that other optimization work. Then some joker writes some C++ code, and the rest of your week is spent writing wrapper functions because your FFI cannot deal with the name mangler.
At the end of the day there is no particular technical reason for C or C++ to remain so popular, and a big pile of technical reasons to stay away from such languages. C made a bit of sense in the 1970s when computers were small and the understanding of compilers and programming languages was less well developed. At this point C and C++ are a liability that we are all stuck with. Maybe some day the expense of sticking with C and C++ will outweigh the expense required to switch to better languages, but I am not holding my breath.
Yes, it comes within a 3x-5x factor, on a good day. And for many applications, that's good enough. However, for either heavy-duty computational tasks or very responsive interactive tasks with a strong computational component, it just doesn't work.
If you have evidence to the contrary (for non-trivial examples), please share it. It's not my love for the exquisite language design that keeps me with C++ :)
* High-frequency trading [1]
* 3D graphics and CAD systems by Symbolics
* Operating systems by Symbolics [2]
* Computer algebra [3]
* Supercomputing [4]
* Embedded, real-time forensic fingerprint systems (fingerprint analysis, embedded databases) [5]
* High-frequency auctions
* Performant compilers (most Lisp compilers)
* Perl-compatible regular expressions (sometimes 2x the speed of perl) [6]
And I can assure you, there are extremely many other things.
Generally, if you write absolutely correct and robust C++ code (that ensures there will never be buffer overruns, integer overflow, etc.), you'll see your code will slow down a lot. Lisp ensures these things don't happen (among many other things), and only when you tell Lisp that you are absolutely sure such things cannot happen, then your Lisp code can and often will be competitive with C or C++.
C++ has also benefitted from corporations funding the research and development of the compilers, whereas Lisp hasn't. So, as a result, the speed is partly an artifact of the implementation, not the language.
Lastly, as my own aside, the supposed "raw speed" of C (and lesser so C++) is no excuse to architect an entire system in it. There are hot paths in code that need speed, and perhaps attention should be given to those.
[1] http://www.hpcplatform.com/
[2] http://en.wikipedia.org/wiki/Genera_(operating_system)
[3] http://maxima.sourceforge.net/
[4] http://en.wikipedia.org/wiki/Connection_Machine
[5] http://arxiv.org/abs/1209.5625
[6] http://web.archive.org/web/20080624164217/http://weitz.de/cl...
Duh, Lisp of course. (Or Haskell.)
> Note that at a bare minimum, these supposed tools should give the ability for fine-grained manual resource control
Check. Lisp provides garbage collection, but you don't have to use it. It's perfectly possible to write Lisp programs that do manual memory management. It isn't often done because it's hardly ever a win, but if you really want to you can.
> zero-(runtime)-cost abstractions
a.k.a. macros
> and performance roughly on-par with C++.
The SBCL compiler is pretty good. But one of the reasons that Lisp code is not generally as fast as C/C++ is that Lisp code is safe by default whereas C/C++ code is not. You can make Lisp code unsafe (and hence faster) but you have to work at it, just as you can make C/C++ code safe, but you have to work at it. I submit that in today's world, being safe and a bit slower by default might not be such a bad place to be in the design space.
> these are all things that tend to be written in C++ or plain C.
There is a world of difference between C++ and plain C. C is actually not a bad language if you want to write fast code with relatively little effort and don't care about reliability or security. The value add of C++ over C is far from clear. (I don't know of any OS written in C++. Linus wrote a famous rant about why Linux is written in C and not C++. There are, however, examples of operating systems written in Lisp.)
> wouldn't it be more productive to ask questions about why people use it?
I know why people use it: it's fast, there is a huge installed base, and it's an excellent platform for studly programmers to display their studliness. That doesn't change the fact that C++ has deep design flaws which result in its being incredibly hard to use and extend. And the existence of coders studly enough to be productive in C++ does not change the fact that it imposes an extremely high cognitive load on its users.
How does LISP work without a garbage collector? Closures without a garbage collector are pretty awful. Let's keep in mind that Rust gives us a pretty good idea of what a safe system without a GC looks like, and it doesn't look anything like LISP.
The same way any other language works without one: you allocate the storage you need and manage it yourself. It's not pretty, but it can be done. The resulting code ends up looking an awful lot like the code in any other imperative language. (Math gets a little tricky because you have to be careful not to inadvertently create bignums, but other than that it's pretty straightforward.)
> Closures without a garbage collector are pretty awful.
> Rust gives us a pretty good idea of what a safe system without a GC looks like
And yet, Rust has closures :-) (And indeed, they are pretty awful.)
Writing non-consing code in Lisp is no different from writing non-consing code in any other language. You can produce stack-allocated closures that get cleaned-up on function return, just as in Rust. If you want to write non-consing code in Lisp (or any other language) you just can't use first-class closures.
(I'd also add that there are some issues surrounding larger unboxed types, but I won't venture to posit how SBCL handles those).
But yes, it's a lot easier to write code with a GC than without one. That's true in any language.
But let's not forget that the original article was saying, essentially, "Hey, look, we can make C++ do Lispy things!". My point is just that if you want to do Lispy things it's a lot easier just to use Lisp than to try to shoehorn Lisp's features into C++.
(It does appear that SBCL lets you drop down to assembly, but again if you do that all the advantages of using LISP are gone. Anyway, what is the goal here? Do you really want to use a typed LISP with no lists, with large of featureless statically allocated memory, manually handling concurrency, mutability everywhere, inline assembly, and the inability to use even most of the C++ LISPy features because LISP has no support for using them without runtime costs? Writing a language without resorting to costly abstractions is hard and it was explicitly never a goal of LISP to be one. That's not to mention that in LISP it's nonobvious which features are costly and which ones aren't, so the abstraction it provides over hardware is only theoretical in this context).
In any LISP in a high performance context, you are always paying for things you don't use. You could probably argue that some of the above problems could be mitigated if everyone adopted SBCL as the standard, but unfortunately that's just the way it is in the real world. And while it is unfortunate, the fact is that even all the technical problems could be resolved (I have my doubts), it would be much more irritating to write such low-level systems code in LISP than in a language that wasn't so far removed from the workings of modern computer architecture.
I'm actually a big fan of Lisp and I've found it quite useful for a number of projects, but when you really need to do low-level programming, it is significantly easier in (modern) C++.
That's right. There's no such thing as a free (no pun intended) lunch.
> (by the way, writing a performant, bugfree, concurrent malloc and free is not that easy :)),
It's pretty easy, actually:
(defvar free-list)
(defun initial-malloc (n) (dotimes (i n) (push (cons nil nil) free-list)))
(defun my-cons (car cdr) (setf (caar free-list) car (cdar free-list) cdr) (pop free-list))
(defun free (cons) (push cons free-list))
The reason it's hard to write a malloc for C is that it has to manage variable-length blocks.
> That's not to mention that LISP has to interact with C on a regular basis for things like system calls, and comes with a runtime that prevents it from playing nicely as an embedded library
No, that's just wrong. There's nothing about Lisp that prevents it from being implemented as an embedded library, e.g.:
http://en.wikipedia.org/wiki/Embeddable_Common_Lisp
> You could probably argue that some of the above problems could be mitigated if everyone adopted SBCL as the standard
No, I'm saying use the right tool for the job. If you really need every last bit of speed and you don't care about safety or engineering cost then by all means use C or C++. But if you want safety, reliability, and the sort of run-time dynamism described in the original article you're better off using Lisp or its progeny.
I'm also saying that if you want performance and you also want to use Lisp, you can. But at the end of the day there are fundamental tradeoffs in computing between speed, safety, dynamism, and engineering cost that no language will save you from.
There are many contexts in which your malloc won't perform well, and many more where it will fall over in a concurrent environment (unless LISP uses atomic operations and locks by default, in which case you have much bigger performance problems to worry about). Concurrency without garbage collection is nontrivial, though I don't blame you for not thinking about it all that much if you rarely interact with such languages. It's great to learn about some of LISP's better-performing utilities (push and pop for example) but let's not get carried away. Also, allocating fixed-length blocks of memory is a perfectly reasonable allocation strategy in C.
> No, that's just wrong.
From the link, embeddable common LISP comes with a runtime, which makes it inappropriate in many contexts. I didn't say that LISP couldn't interact with C (obviously it can!) only that it's not particularly convenient. From the link, it supports inline C, which is great, but again you're not really using LISP at this point.
> No, I'm saying use the right tool for the job.
Oh, sure, I don't think we're disagreeing on that. Certainly most of the prominent Rust developers will immediately point you to a language like Haskell, Nimrod or Python if they will satisfy your usecase. It's just that some of your posts suggested that you think LISP could in principle be used in all the places C++ is used, which I don't think is necessarily true, and certainly it wouldn't be convenient to do so. For people who do have to use C++, I think these LISPy features are a nice way to make the experience more tolerable, and I think that's all the article was getting at.
Good point (but you have that problem in any language). However, PUSH conses, so my code is wrong in that regard (you have to use a pre-allocated free vector, not a free list). So I concede the point: writing your own allocator in Lisp is not trivial. But it can be done.
> I don't think we're disagreeing on that.
Let's just leave it at that for now then.
[0] Granted, that's an "operating system framework", but it's definitely at the same "level" as implementing an OS.
No, macros are not what I'm talking about here: I mean that C++ provides abstractions that only impose runtime costs if you use them. For instance, the cost of vtable lookup is only paid if you are using virtual functions; otherwise, you don't have any overhead for function calls beyond what's imposed by the hardware.
As noted elsewhere in the thread:
> Unlike Lisp, C++ lets one write efficient, generic algorithms that can operate on several types of data. Lisp cannot, and falls back to dynamic type testing, which makes for slower code[1]. Basically your only option in Lisp is to specialize everything manually, or inline everything. Both approaches are extremely poor. As I hinted, Haskell and Standard ML do an even better job than both C++ or Lisp. This is talked about a good bit in this article [2].
That's what I mean when I say 'zero-cost abstraction'.
Another sign of the lack of coherence of the article is that it displays unoptimized assembly output for the C++ example, then goes on to praise Lisp for being around as good. The optimal assembly is actually two instructions:
0000000000000000 leal 0x1(%rdi), %eax
0000000000000003 ret
mov eax, 1
add eax, [esp + 4]
retWhat's interesting from a language perspective is that the C++ folks seem to take the approach of "ugly is better" by essentially not breaking backwards compatibility like Python 3 and Perl 6 did. The "clean break" approach seems to create a new species and hope that the old one goes extinct (didn't happen with Perl, probably won't). But C++ just keeps evolving.
And, for all the smug superiority of the Lisp crowd, the niche that C++ is trying to occupy does seem to be significantly bigger than the Lisp niche...
Specialization is when you tell the compiler to use a separate implementation of a template when some or all of the template arguments match something. Template specialization is used in the type traits section of the post.
While it will take a lot more that just this book to get anywhere I think its a good starting point for anyone with experience with other programming languages who want to see what C++ has to offer.
Personally, if you aren't new to programming per se, I'd go with "C++ Primer" by Lippman/Lajoie/Moo since it smoothly integrates modern C++11 throughout the entire text (instead of sticking it into a separate section, as some of the other books do).
After that, "C++ Concurrency in Action: Practical Multithreading" by Anthony Williams: http://www.manning.com/williams/
...and then the rest of the books from the isocpp list (e.g., Josuttis).
1. Libraries:
The rich ecosystem of available libraries is one of my primary reasons for using C++ for numerics :-)
In fact, it's rich enough that it may be best if you were to specify what kind of number crunching you're interested in -- right now I can only try to give you a very broad/big-picture list of some that I've found useful.
The Standard Library supports (P)RNG with a variety of statistical distributions: http://en.cppreference.com/w/cpp/numeric/random
- Boost.Math Toolkit: http://boost.org/libs/math // and more broadly: http://boost.org/doc/libs/?view=category_Math // and even more broadly ;-): http://www.boost.org/doc/libs/?view=categorized - Eigen: http://eigen.tuxfamily.org/ - GPGPU: http://www.soa-world.de/echelon/2014/04/c-accelerator-librar... - MLPACK: http://mlpack.org/ - NLopt: http://ab-initio.mit.edu/wiki/index.php/NLopt_C-plus-plus_Re... - OpenCV: http://opencv.org/ - Odeint: http://www.odeint.com/ - POCO: http://pocoproject.org/ // note: not numerics, but when you need to exchange data over the net/web, these are pretty good for that :-) - QuantLib: http://quantlib.org/ // note: QuantLib is primarily for quantitative finance, but also has math components: http://quantlib.org/reference/group__math.html - SOCI: http://soci.sourceforge.net/ // note: not numerics, but for when you need database access, it has pretty clean API and is easy to use :-)
2. Talks:
* C9 Going Native: http://channel9.msdn.com/Shows/C9-GoingNative
In particular: + "Bjarne Stroustrup - The Essence of C++: With Examples in C++84, C++98, C++11, and C++14" - http://channel9.msdn.com/Events/GoingNative/2013
+ "Sean Parent - C++ Seasoning" - http://channel9.msdn.com/Events/GoingNative/2013/Cpp-Seasoni...
* BoostCon / C++Now!: https://github.com/boostcon/
There's _lots_ of interesting talks, so explore yourself :-)
For instance, 2013 Keynote: "Dan Quinlan: C++ Use in High Performance Computing Within DOE: Past and Future" // http://2013.cppnow.org/session/keynote/
// IMHO, it's worth watching these for staying up to date with the broader developments in the field -- e.g., according to the speaker (given who he is I'd assume credibility) most national labs, including Lawrence Livermore National Laboratory in particular, are quite actively adopting C++ (not C) and have been turning away from Fortran for some time now.
HTH! :-)
(To answer your question: linear algebra and optimization. I do a lot of Numpy right now, and I see Eigen in my future...)