Monday, February 27, 2012

Confessions of a Lisp Hater

I hate to admit this, but sometimes I find myself missing python. For example, the other day, I wrote this horrible mess^H^H^H^H^H^Hbeautiful example of templated C++:
CollectionVisitor<Pmwx,Face_handle,UnlinkedFace_p> face_collector(&friends, UnlinkedFace_p(&io_faces));
VisitAdjacentFaces<Pmwx, CollectionVisitor<Pmwx,Face_handle,UnlinkedFace_p> >(*f, face_collector);
Once you start writing really well-factored C++ template functions, eventually you'll reach this point: the verbosity of code is dominated by having to write a ton of function objects because C++ doesn't have closures. (Yes, closures. No one is more pained to admit envy of a Lisp feature than me.)

What makes the above code particularly ugly isn't just, um, that mess, but the other shoe - the UnlinkedFace_p predicate is several lines of boiler plate for about one expression of useful C++:
struct UnlinkedFace_p {
set<Face_handle> * universe_;
UnlinkedFace_p(set<Face_handle> * universe) : universe_(universe) { }
bool operator()(Face_handle who) const {
return universe_->count(who) > 0;
(Note: I am trying to get the less than and greater than symbols manually converted so Blogger doesn't delete 3/4 of my templated code like it has in the past.)

What makes this function so verbose is that the "closure" - that is, the act of copying the locally scoped data "into" the predicate object has to be done by hand for every predicate we write, and the predicate can't be located in the function where it really belongs. (At least, if I try to locate my struct inline, GCC becomes quite cross. There may be fine print to get me closer to predicates inline with the code.)

In Python this would be a lot more pleasant. There's no typing, so strip out all of that type-declaring goo, and we have real closures, so the entire function chain can be written as one nested set of calls that contain pretty much only the good bits.

This got me thinking about the difference between coding in C and Python. Here comes another stupid coding quote:
Coding in C is like going out to dinner with someone who's really cheap and insists on discussing the price of everything on the menu. "You know, that pointer dereference includes a memory access - 12 to 200 cycles. That ? involves conditional code, you might mis-predict the branch. That function is virtual - you're going to have two dependent memory fetches." You know the cost of everything on the menu. 
Coding in Python is like going on a shopping spree with your friend's credit card. "Go ahead, iterate through a list of lists, and insert in the middle. If it feels right, just do it. No, it's not expensive, why do you ask?"

Friday, February 17, 2012

XCode, Lion, GCC Oh My!!

My curiosity always pushes me to upgrade to the latest OS despite the caution issued to me by Ben. I upgraded to Lion and quickly found that XCode 3.2.6 cannot be installed on Lion. We're not ready to move completely to XCode 4 just yet so I had to find a work around. Luckily, there is one.

Basically, you just have to follow these simple steps:
  • Mount the Xcode 3.2.6 DMG
  • Open Terminal
  • Enter the commands:
    open "/Volumes/Xcode and iOS SDK/Xcode and iOS SDK.mpkg"
Because I need to run XCode 4 and XCode 3.2.6 side by side, I installed XCode 3.2.6 to /Developer-3.2.6 and let XCode 4 have the default /Developer directory.

Ok that gets me a working copy of XCode 3.2.6 as well as the SDKs that I need which are 10.5 and 10.6...but I'm not out of the woods yet because the X-Plane Scenery Tools requires the 10.5 SDK. I have it so there's no problem right? WRONG! The GCC being used by the system is in /Developer and if you take a look at /Developer/SDKs you'll see 10.6 and 10.7 but no 10.5. That's because 10.5 lives in /Developer-3.2.6 but GCC won't be looking there. My solution was to create a symbolic link (the linux kind ln -s, not the Mac Alias kind) in the /Developer/SDKs folder that points to the /Developer-3.2.6/SDKs folder. The actual command was:

ln -s /Developer-3.2.6/SDKs/MacOSX10.5.sdk /Developer/SDKs/MacOSX10.5.sdk

So now with that done, GCC now has access to the right SDKs. The finder view of /Developer/SDKs should look like this:

At this point, I tried to compile the libs necessary for the X-Plane Scenery Tools again but was hit with one more snag as it was failing to find bits/c++config.h as referenced by iostream.h and a dozen other files. It turns out that the new GCC is a darwin11 version of the compiler. That's fine but if you look at /Developer/SDKs/MacOSX10.5.sdk/usr/include/c++/4.2.1/ you'll notice that there are several folders that are named with respect to the GCC version they're designed for...the ones we care about are:
  • i686-apple-darwin*
  • x86_64-apple-darwin*
Here's a screenshot of what it looks like on my system:

You'll notice that there's no version for darwin11 which is why the headers cannot be found. The solution once again is a symbolic link. You'll notice that darwin8 and darwin10 are actually really links to darwin9. All that I had to do now is create a symbolic link back to version 9 for the 32bit and 64bit paths. Here's the commands:

ln -s i686-apple-darwin9 i686-apple-darwin11
ln -s x86_64-apple-darwin9 x86_64-apple-darwin11

After these steps, the universe seemed to be back in order again. XCode 4 and XCode 3.2.6 co-exist properly and the scenery tools compile using the newer version of GCC even using the older 10.5 SDK.