What's the canonical way to do partial rebuilds?

[bcherny]

I have a directory of files, which I am compiling 1-to-1:

in:

src/
|_ foo/
    |_ foo.coffee
|_ bar.coffee

out:

dist/
|_ foo/
    |_ foo.js
|_ bar.js

What's the right way to watch and do partial rebuilds when a file changes (ie. only recompile the changed file)? An example would be nice.

[yocontra]

https://github.com/gulpjs/gulp#incremental-builds