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Void
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When an expression is not in a value context, it is given type Void.
For example, in the expression ::

  r := (a; b; if c then d else e; f) 

values are used only from the subexpressions c and f: all others are thrown 
away.  The subexpressions a, b, d and e are evaluated for side-effects only 
and have type Void.  There is a unique value of type Void.

You will most often see results of type Void when you declare a variable. ::

  a : Integer
                                Type: Void

Usually no output is displayed for Void results.  You can force the
display of a rather ugly object by issuing ::

  )set message void on

  b : Fraction Integer
                                Type: Void

  )set message void off

All values can be converted to type Void. ::

  3::Void
                                Type: Void

Once a value has been converted to Void, it cannot be recovered. ::

  % :: PositiveInteger
   Cannot convert from type Void to PositiveInteger for value "()"

See Also:

* )show Void