side_effect.er

if True, () -> log "hello"
if! True, () => print! "hello"
# if True, () => print! "hello" # this should cause a type error
if True, do:
    _x = "aaa" + input!() # this should cause an effect error
    print! "hello" # this should cause an effect error

f x: Int = log x
g x: Int = print! x # this should cause an effect error

echo = print! # this should be an effect error
_echo = # this is OK
    print! 1
    log