It might not be the most mind-bending, but it has been by far the hardest for me to learn(I thought Haskell and assembly were easier!) 

That is the set of HDL languages, notably, VHDL (and verilog to a lesser extent) 

Getting over the fact that every "function" runs at the same time is incredibly difficult and you can not avoid the shift to parallelizing absolutely everything. Of course, this is only border-line a programming language though.