Nested functions in GCC

GCC supports “nested functions” using the -fnested-functions flag. When I first saw this, I was excited: closures in C! In the famous words of Admiral Ackbar, “it’s a trap!”


typedef int (*fptr)(int);

fptr f(int arg) {
  int nested_function(int nested_arg) {
    return arg + nested_arg;

  return &nested_function;

void smash(int arg) {

int main(void) {
  fptr g = f(10);
  printf("%d\n", (*g)(5));
  // printf("%d\n", (*g)(5));
  fptr h = f(12);
  printf("%d\n", (*g)(5));
  printf("%d\n", (*h)(5));

  return 0;

Try compiling (gcc -fnested-functions). What does the second call to g produce—15 or 17? Try uncommenting line 21. What happens? Does commenting out line 20 affect this? What if line 19 is commented out, but lines 20 and 21 are uncommented?

I’m not sure this feature is worth it.

Locally installing LLVM with Ocaml bindings

We can’t install software into the /usr tree at my office, so I end up having local installs of lots of software. Some things, like GODI, play well with this. I had some trouble finding the right way to get LLVM‘s Ocaml bindings to work, so I figured I’d share the wealth. The following instructions will put an install into the directory $PREFIX/llvm-install.

Here are the steps; they’re followed by a plain English explanation.

svn co llvm
tar xzf llvm-gcc4.2-2.5-x86-linux-RHEL4.tar.gz
mkdir llvm-objects llvm-install
cd llvm-objects
../llvm/configure --with-llvmgccdir=$PREFIX/llvm-gcc4.2-2.5-x86-linux-RHEL4 --enable-optimized --enable-jit --prefix=$PREFIX/llvm-install --with-ocaml-libdir=$GODI_PATH/lib/ocaml/std-lib
make install

My PREFIX is my home directory, and GODI_PATH = ~/godi. First, we checkout the latest LLVM from SVN (step 2). Then we download and extract the latest release (2.5, as of writing) of LLVM-gcc (steps 3 and 4). (I couldn’t get the SVN version of LLVM-gcc to work with the SVN version of LLVM.) Notably, LLVM does not support in-place builds, so we create the llvm-objects directory to actually build LLVM; we’ll install it into llvm-install (step 5). We configure the software from the llvm-objects directory (steps 6 and 7). The long configure is necessary; the only optional item is --enable-jit. You may have to adjust your --with-ocaml-libdir to point to wherever your Ocaml libraries live. Then make and make install (steps 8 and 9). Voila!

To test it out, we can use the “Hello, World!” program written by Gordon Henrikson. I had to change it a little to bring it up to date with the latest APIs (in particular, the global context had to be added). You can download it as

open Printf
open Llvm

let main filename =
   let c = create_context () in

   let i8_t  = i8_type c in
   let i32_t = i32_type c in

   let m = create_module c filename in

   (* @greeting = global [14 x i8] c"Hello, world!\00" *)
   let greeting =
     define_global "greeting" (const_string c "Hello, world!\000") m in

   (* declare i32 @puts(i8* ) *)
   let puts =
     declare_function "puts"
       (function_type i32_t [|pointer_type i8_t|]) m in

   (* define i32 @main() { entry: *)
   let main = define_function "main" (function_type i32_t [| |]) m in
   let at_entry = builder_at_end c (entry_block main) in

   (* %tmp = getelementptr [14 x i8]* @greeting, i32 0, i32 0 *)
   let zero = const_int i32_t 0 in
   let str = build_gep greeting [| zero; zero |] "tmp" at_entry in

   (* call i32 @puts( i8* %tmp ) *)
   ignore (build_call puts [| str |] "" at_entry);

   (* ret void *)
   ignore (build_ret (const_null i32_t) at_entry);

   (* write the module to a file *)
   if not (Llvm_bitwriter.write_bitcode_file m filename) then exit 1;
   dispose_module m

let () = match Sys.argv with
  | [|_; filename|] -> main filename
  | _ -> main "a.out"

Now we can compile:

ocamlopt -cc g++ llvm.cmxa llvm_bitwriter.cmxa -o llvm_test
./llvm_test hello.bc # generates bitcode
$PREFIX/llvm-install/bin/llvm-dis hello.bc # disassembles bitcode into hello.ll
$PREFIX/llvm-install/bin/lli hello.bc # outputs "Hello, world!"

If interpretation via lli isn’t your bag, you can also compile to native code:

$PREFIX/llvm-install/bin/llc hello.bc # generates assembly, hello.s
gcc -o hello hello.s
./hello # outputs "Hello, world!"

PHPEnkoder 1.6

Martin Rees caught another bug in PHPEnkoder, which was making it difficult to edit posts with comments containing e-mails. This problem has been solved by turning off the enkoder filters when displaying administrative panels.

In addition to the bugfix, there are two improvements. First, the internal enkoding system will choose names that are more likely to be unique. Second, I’ve added a shortcode, enkode. You can use it to manually enkode an arbitrary stretch of text, like so: [enkode]this will be enkoded[/enkode].

The latest version is available from the PHPEnkoder website and its home in the plugin directory.

Flapjax: A Programming Language for Ajax Applications

I am immensely pleased to report that our paper on Flapjax was accepted to OOPSLA 2009.

This paper presents Flapjax, a language designed for contemporary Web applications. These applications communicate with servers and have rich, interactive interfaces. Flapjax provides two key features that simplify writing these applications. First, it provides event streams, a uniform abstraction for communication within a program as well as with external Web services. Second, the language itself is reactive: it automatically tracks data dependencies and propagates updates along those data?ows. This allows developers to write reactive interfaces in a declarative and compositional style.

Flapjax is built on top of JavaScript. It runs on unmodi?ed browsers and readily interoperates with existing JavaScript code. It is usable as either a programming language (that is compiled to JavaScript) or as a JavaScript library, and is designed for both uses. This paper presents the language, its design decisions, and illustrative examples drawn from several working Flapjax applications.

The real heroes of this story are my co-authors. Leo, Arjun, and Greg were there for the initial, heroic-effort-based implementation. Jacob and Aleks wrote incredible applications with our dog food. Shriram, of course, saw the whole thing through. Very few of my contributions remain: the original compiler is gone (thank goodness); my thesis work is discussed briefly in How many DOMs? on page 15. Here’s to a great team and a great experience (and a great language)!

PHPEnkoder 1.5

Martin Rees noticed that any user can change PHPEnkoder’s settings. I’ve change PHPEnkoder’s settings panel to require the manage_options capability. Now, by default, only administrators can change PHPEnkoder’s settings. (If you’re unfamiliar with the concept, check out the Codex documentation on roles and capabilities.)

As usual, the plugin is available from the PHPEnkoder website and its home in the plugin directory.

Debounce and other callback combinators

It is serendipitous that I noticed a blog post about a callback combinator while adding a few drops to the Flapjax bucket.

Flapjax is nothing more than a coherent set of callback combinators. The key insight to this set of callback combinators is the “Event” abstraction — a Node in FJ’s implementation. Once callbacks are Nodes, you get two things:

  1. a handle that allows you to multiply operate on a single (time-varying) data source, and
  2. a whole host of useful abstractions for manipulating handles: mergeE, calmE, switchE, etc.

The last I saw the implementations of Resume and Continue, they were built using this idea. The more I think about it, the more the FJ-language seems like the wrong approach: the FJ-library is an awesome abstraction, in theory and practice.

PHPEnkoder 1.3

Ron Blaisdell pointed out that my use of noscript elements wasn’t XHTML compliant. Instead of using noscript tags, each enkoded section is preceded by a span containing the “you don’t have JavaScript” message. When the dekoded text is written to the document, this span is deleted.

The latest version is up on PHPEnkoder’s home page and the WordPress plugin directory. (For some reason, PHPEnkoder doesn’t come up when you search for it in the directory, but Google can see it. I’m not sure what the problem is here…)