▁Language , ▁but ▁is ▁convention ally ▁sp elled ▁" T cl " ▁rather ▁than ▁" T CL ". ▁ ▁T cl ▁con ferences ▁and ▁work sh ops ▁are ▁held ▁in ▁both ▁the ▁United ▁States ▁and ▁Europe . ▁ ▁Fe atures ▁ ▁T cl ' s ▁features ▁include ▁▁ ▁All ▁operations ▁are ▁commands , ▁including ▁language ▁structures . ▁They ▁are ▁written ▁in ▁prefix ▁notation . ▁ ▁Comm ands ▁are ▁commonly ▁vari adic . ▁ ▁Everything ▁can ▁be ▁dynamically ▁re defined ▁and ▁overrid den . ▁Actually , ▁there ▁are ▁no ▁keywords , ▁so ▁even ▁control ▁structures ▁can ▁be ▁added ▁or ▁changed , ▁although ▁this ▁is ▁not ▁advis able . ▁ ▁All ▁data ▁types ▁can ▁be ▁manip ulated ▁as ▁strings , ▁including ▁source ▁code . ▁Intern ally , ▁variables ▁have ▁types ▁like ▁integer ▁and ▁double , ▁but ▁converting ▁is ▁purely ▁automatic . ▁ ▁Vari ables ▁are ▁not ▁declared , ▁but ▁assigned ▁to . ▁Use ▁of ▁a ▁non - defined ▁variable ▁results ▁in ▁an ▁error . ▁ ▁Ful ly ▁dynamic , ▁class - based ▁object ▁system , ▁T cl O O , ▁including ▁advanced ▁features ▁such ▁as ▁meta - classes , ▁filters , ▁and ▁mix ins . ▁ ▁Event - dri ven ▁interface ▁to ▁so ckets ▁and ▁files . ▁Time - based ▁and ▁user - defined ▁events ▁are ▁also ▁possible . ▁ ▁Variable ▁visibility ▁restricted ▁to ▁lex ical ▁( static ) ▁scope ▁by ▁default , ▁but ▁u ple vel ▁and ▁up var ▁allowing ▁pro cs ▁to ▁interact ▁with ▁the ▁en clos ing ▁functions ' ▁scop es .