add typechecker rule
add operator <<| and <.| ; tidy-up codes and commments
tidy-up codes
tidy-up codes
add '=:='; add 'error'; tidy-up codes
rename eval -> check_type_of; tidy-up codes
define '<<$', '<.$', 'join', 'dash' functions
refactor module action; tidy-up codes
refactor codes
refactor codes
tidy-up codes