Higher-kinded endofunctors

Higher-kinded type classes is what makes Haskell stand out above other functional languages for me. Things like endofunctors just cannot exist in virtually any other language.

immutable data

So to replace all 'a's 'b's and 'c's you need to malloc 3 strings?

referentially transparent functions

pass-by-reference is immensely useful,pass-by-value is introducing copies and extra variables.
using globals and global state is the most efficient way.