MLton Standard ML Compiler (SML Compiler)

MLton Standard ML Compiler (SML Compiler) - MLton is required for Ur/Web, and conveniently, there is a binary download for OS X that works quite well if you have the GnuMP library installed via MacPorts (you can also install the mhash library from there, which is another prerequisite for Ur/Web). MLton is also an interesting project in its own right, as it is a standalone compiler for ML that has very good optimizations (though it also has extreme compilation times for the compiler).