R environments and zombies

“Oh no! It’s all the R functions that melt your brain in one line all together! What’s going on here is that R function arguments are not actually fully evaluated, but rather, they contain “promises.” So, before the argument is used as a value, you can actually grab the symbolic expression, convert it to text or even reevaluate it in another context. It’s like reaching back up the call stack with your zombie programmer hand to eat the brains of the code that called you.”

– R environments and argument passing (or, My God! It’s full of LISP!)

(via Jana)