Here it is in Erlang :)

caesar.erl

-module(caesar).

-export[cipher/2].

cipher(L, Shift) when is_list(L), is_integer(Shift), Shift =< 25, Shift >= -25 ->

   %% Anonymous function with guards. 
   %% Strings are lists of numbers in Erlang [97,98,99] = [$a,$b,$c] = "abc"
   %% We evaluat X and add it's shifted value to the head of the accumulator list Acc.

   F = fun(X,Acc) when X + Shift  > $z ->
              [X + Shift - 26|Acc];
          (X,Acc) when X + Shift < $a ->
              [X + Shift + 26|Acc];
          (X,Acc) ->
              [X + Shift|Acc] 
   end,

  
   lists:reverse(lists:foldl(F,[],L)).  %% foldl is tail recursive, and lists:reverse is an optimized C BIF.

>>>>>>

Eshell V5.9.3.1  (abort with ^G)
1> c(caesar).
{ok,caesar}
2> caesar:cipher("cxyyblvhhe",7).
"jeffiscool"
3> caesar:cipher("jeffiscool",-7).
"cxyyblvhhe"
4>