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pjmlp

Lisp does support more than just lists as data structures.

Arrays are also available, including specialized versions that hold value types.

https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node15.html

You can also stack allocate if required, via dynamic-extent, http://clhs.lisp.se/Body/d_dynami.htm

Also not all Lisps have a tracing GC, some variants had a RC with tracing GC for collecting cycles.

RAII like patterns can be achieved via the with-.... functions, or macros.

I don't know the actual performance of commercial Lisps like Allegro Common Lisp and LispWorks, but I imagine it is quite good, given that they stay in business.

On the other had, given the amount of money spent in C and C++ optimizers vs the lack of industry wide adoption of Lisp, probably still not as good as current leading C++ compilers.

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Indeed, those with- macros.

In TXR Lisp, RAII is supported thusly:

This is GC finalization of a struct:

  This is the TXR Lisp interactive listener of TXR 172.
  Use the :quit command or type Ctrl-D on empty line to exit.
  1> (defstruct animal nil
       (:fini (me) (put-line `@me says good-bye`)))
  #<struct-type animal>
  2> (progn (new animal) nil) ;; make animal without referencing from REPL
  nil
  3> (+ 2 2)
  4
  4> (sys:gc)
  #S(animal) says good-bye
  t

OK, now with-objects macro:

   5> (with-objects ((a (new animal)))
       (prinl a))
   #S(animal)
   #S(animal) says good-bye
   #S(animal)

with-objects invokes finalizers explicitly, before objects become unreachable.

Also, what if a constructor throws? Let's derive animal to dog which bails at `new` time:

  6> (defstruct dog animal
       (:fini (me) (put-line `@me: woof woof`))
       (:postinit (me) (error "refuse to construct")))
  #<struct-type dog>
  7> (new dog)
  #S(dog): woof woof
  #S(dog) says good-bye
  ** refuse to construct
  ** during evaluation at expr-6:3 of form (error "refuse to construct")

The object instantiation logic catches exceptions and invokes finalizers on a partially constructed object (in the proper order as you can see: derived, then base).

The with-* style function do not replace RAII except for the limited case of lexically scoped resources. Not all resources are scoped lexically -- e.g. file handles you store in a map. What C++ programmers typically see as the most important guarantee is that RAII is prompt freeing -- that is, as soon as said example map goes out of its scope (or is freed by its owner), then those file handles will also be freed immediately.

Neither the gc-based nor with-* based solutions handle that.

Even on C++ RAII requires a lexical scope at some point in the program.

Somewhere in the whole workflow there must be a class allocated in the stack.

You can have destructor like behavior in Lisp as well, just register a file handle cleanup action by giving a cleanup lambda when creating the map instance.

The function that removes entries from the map will call the provided lambda.

I'm sure I must be missing something, but can you give a concrete example?

Of course instances of classes (not "classes", as you say, classes are an entirely compile-time construct in C++) must be allocated, but I mean... there's still heap storage. You know, shared_ptr<T> and all that...

What am I missing?

> You can have destructor like behavior in Lisp as well, just register a file handle cleanup action by giving a cleanup lambda when creating the map instance.

Well, except you don't know exactly when the map is going to get cleaned up...? And it could get reused across various sections of the program... Do you see what I'm talking about?

EDIT: That, and if the map exists for the entire duration of the program, but it's really important that entries have prompt cleanup when removed... what then?

(Btw, I do understand that in the general case with weak_ptr, shared_ptr, unique_ptr, etc. that things get decidedly less deterministic[1], but RAII is pretty well defined by scope or referenced-to-scope.)

[1] Basically almost as unpredictable as a general purpose GC. I can't recall the paper title, but I'm sure there is a paper out there detailing this.

TXR Lisp:

  This is the TXR Lisp interactive listener of TXR 173.
  Use the :quit command or type Ctrl-D on empty line to exit.
  1> (defstruct map nil
       (hash (hash :equal-based))
       (:method lambda (me key) (gethash me.hash key))
       (:method lambda-set (me key new-val)
         (let ((old-val (shift (gethash me.hash key) new-val)))
           (call-finalizers old-val)
           new-val))
       (:fini (me)
         (dohash (key val me.hash)
           (call-finalizers key)
           (call-finalizers val))))
  #<struct-type map>
  2> (defstruct widget nil
       id
       (:static counter 0)
       (:init (me) (set me.id (inc me.counter)))
       (:fini (me) (put-line `widget @{me.id} says bye`)))
  #<struct-type widget>
  3> (defvar map (new map))
  map
  4> (set [map "a"] (new widget))
  #S(widget id 1)
  5> (set [map "a"] (new widget))
  widget 1 says bye
  #S(widget id 2)
  6> (set [map "a"] (new widget))
  widget 2 says bye
  #S(widget id 3)
  7> (set [map "b"] (new widget))
  #S(widget id 4)
  8> (set [map "c"] (new widget))
  #S(widget id 5)
  9> (call-finalizers map)
  widget 3 says bye
  widget 4 says bye
  widget 5 says bye
  10> (with-objects ((m (new map)))
        (set [m "x"] (new widget))
        (set [m "y"] (new widget)))
  widget 6 says bye
  widget 7 says bye
  #S(widget id 7)

You can see in my example that non-lexically scoped resources are involved also.

Secondly, doing RAII into and out of a map container is largely a poor idea, and C++ programmers expend energy fighting it.

The assignment operation and constructors and destructors of the map element all have lexical scopes somewhere.

> You can see in my example that non-lexically scoped resources are involved also.

Please explain your example further then, because I don't see it.

> Secondly, doing RAII into and out of a map container is largely a poor idea, and C++ programmers expend energy fighting it.

Again, explain, please. Why is it a "poor idea"? Any links, at least? "Fighting it"... how, exactly?

(It's been a while since I was a professional C++ programmer, but I don't really recognize this characterization.)

> The assignment operation and constructors and destructors of the map element all have lexical scopes somewhere.

Well, yes, but what about the MAP itself? It could easily be heap allocated.

I'm sorry, I just don't see what you're on about.

The map being heap allocated somewhere means this, in its most basic form:

  {
    map<whatever> *ptr = new map<whatever> ;

    // forgotten delete
  }

If you don't want the heap allocated map to leak, and you want that management to be automated, you need some smart pointer type to reference it; map itself doesn't help.

All RAII-driven management of heap resources is tied to some scope somewhere. Anything not tied to scoping has to be treated explicitly: try to calculate the lifetime and explicitly dispose of the resource.

AFAICT you're saying that RAII = RC. Am I correct in thinking that?

"I don't know the actual performance of commercial Lisps like Allegro Common Lisp and LispWorks, but I imagine it is quite good, given that they stay in business."

It might be good or just OK on modern hardware. I imagine they stayed in business for their IDE's, libraries, commercial support, and decent compilers. Batteries included. Some success stories make me think performance is really good, though.

http://www.lispworks.com/success-stories/netfonds-primetrade...

http://www.lispworks.com/success-stories/ral-siglab.html

I found those while looking for one about a real-time implementation for telecom or something. Franz's success stories are mostly about doing complicated stuff easier. There was one that looked performance-critical:

http://franz.com/success/customer_apps/finance/ravenpack.lht...

Here is a another one, a Portuguese company specialized in Lisp based solutions, for timetables and fleet management.

http://www.siscog.eu/

The last time I checked, the naive Lisp will be shorter than the typical C++, but usually much slower. Sometimes you can annotate your code to death for performance, resulting in something longer than the C++ with more parens, but you still can't trust the compiler to get it right. Fortunately, thanks to DEFMACRO, Lisp lets you write your own optimizing compiler, so as long as your implementation supports your full instruction set, you can do that and achieve better-than-C performance by generating Fortran IV's ugly cousin.

> I don't know the actual performance of commercial Lisps like Allegro Common Lisp and LispWorks, but I imagine it is quite good, given that they stay in business.

Performance is not one easy number. Applications have different performance requirements. In many benchmarks for typed or type inferred code SBCL tends to be slightly faster with less programming effort.

Fast can mean for example:

* fastest possible execution of optimized code

Then one might not care about code size, code safety, robustness, threading capability, interrupts, .

* fast execution of non-optimized robust, flexible, reflective, debuggable code.

Allegro CL and LispWorks integrate a lot of features. These features are provided over some amount of platforms, with only few restrictions. They provide relatively good performance of a native code compiler. Some robustness is needed for commercial applications, thus I would expect an advantage there.

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