Code:
alias -l list.filter {
  if ($1) %list.filter = $iif(%list.filter,$v1 $+ $chr(44) $1,$1)
}

alias _list {
  set %list.filter
  filter -k list.txt list.filter
  echo -ag the list contains the ff:
  echo -ag %list.filter
  unset %list.filter
}


/_list

Note that it's subject to the line too long error. It also disregards blank lines in the file.