You mean like Ksh's discipline functions?

  dualbus@system76-pc:~$ ksh -c 'date=; date.get() { .sh.value=$(date +%s); }; echo $date; sleep 5; echo $date'
  1546926637
  1546926642

See: https://docs.oracle.com/cd/E36784_01/html/E36870/ksh-1.html ("Discipline Functions")