isnull

isnull <value>

Whether a value is null — the absence of any value.

This is the exact complement of exists: [isnull $x] is the same as [not [exists $x]]. Built-ins that may fail to find something — first, nth, carrier, object, mob — return null, and isnull is the natural way to detect that.

Arguments

Returns

A bool — true when the value is null (or a stale entity reference), false otherwise.

Examples

after command (say) {
  let target [first [creatures $self]]
  if [not [isnull $target]] {
    do "smile $target"
  }
}

The body runs only when the room holds a creature; if first returns null the if block is skipped (see Control flow). Note that unless is a handler guard and takes no body — use if [not ...] for a conditional block.

after command (say) {
  let proto [object 999999]
  do "say isnull=[isnull $proto]."
}

There is no object prototype with vnum 999999, so object returns null and [isnull $proto] is true.

See also