Is R, lisp and prolog hard?
Objectively speaking, Prolog isn't hard. It just feels hard because most who decided to venture into the language started off with something mainstream, most likely procedural in nature. Lambda Calculus, which forms the base of logical programming, is equivalent in power when compared to the Turing machine, the base of procedural programming. Any problem which can be solved, or anything which can be created, in either paradigm, can be solved or created using the other. It is really the transition from practicing procedural programming to practicing logical/functional programming, that is hard. To someone who started off with Prolog, languages like Java, C, can be as hard.
R easy. Looks like a C/Java derivative Lisp is a language which implements the functional paradigm, to take a full power of lisp you need to think is functions, arguments, lambda calculus etc. And the syntax is almost extraterrestrial. Prolog is not a general purpose language. It implements the Logical paradigm and the statements obey the rules to describe predicate's logic, inference machine first order logic, theorem proofing. A little bit harder to program anything useful out of logic