Ruby is not the only language with "unless", actually. For example it exists in GNU Guile as well (https://www.gnu.org/software/guile/manual/html_node/Condition

Ruby is not the only language with "unless", actually. For example it exists in GNU Guile as well (https://www.gnu.org/software/guile/manual/html_node/Conditionals.html https://www.gnu.org/software/guile/manual/html_node/Conditio...). However, due to the usual structure of functions and the last expression being the return value, one always expects a return value (why else call the function?). So "when" and "unless" are only seen inside procedures, which have side effects.