Would you have any example of what your trainees stumbled upon or found difficult in the language? That would be useful I guess when we try to onboard new people on the project.
Have a nice day :)
Ah, you're mixing up Clojure and Lisp. The restrictions in Clojure have nothing to do with Lisp tradition.
Someone used to a Lisp-like language will also struggle with Clojure.
Traditional Lisps like Common Lisp are multi-paradigm languages in which variables and most aggregate types are mutable.
They are not hard to pick up for someone with a background in C.
C has pass-by-value functions; so does proper, traditional Lisp.
C has mutable variables, arrays and structures; so does Lisp. Lisp data structures like lists or hash tables are easily understandable by a C programmer with a good background in data structures.
cons is like malloc-ing and filling-in a two-field struct, except that freeing is taken care of.
People have written Lisp systems in C (and written more than one book on that exact subject: Nils M. Holm just put a new one out: http://www.t3x.org/lsi/index.html), with a lot of the internals being written in C over the Lisp library functions. E.g. visiting and printing the elements of a list can literally look like this, in the internals of a C-based Lisp implementation:
for (val iter = list; list; list = cdr(list))
print(car(list));
I disagree on that point. In my experience as a programming language teacher, people often struggle to learn functional programming and even more to learn a Lisp, especially if they are used to a C-like language. And learning both at the same time is even more difficult. I liked working with Clojure but I have yet to meet someone who easily grasps the language and - more important - development patterns.