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Rosetta Example: Abstract type

Define a class without instances and without implemented methods. For detailed description of this example see https://rosettacode.org/wiki/Abstract_type 

package req nx

Define a class AbstractQueue

nx::Class create AbstractQueue {

  :public method enqueue {item} {error "not implemented"}
  :public method dequeue {} {error "not implemented"}

  :public object method create {args} {
    error "Cannot instantiate abstract class [self]"
  }
}

Define a concrete queue (named ListQueue) based on the Abstract Queue

nx::Class create ListQueue -superclass AbstractQueue {

  :variable list {}

  :public method enqueue {item} {
    lappend :list $item
  }

  :public method dequeue {} {
    set item [lindex ${:list} 0]
    set :list [lrange ${:list} 1 end]
    return $item
  }
}

Demonstrating the behavior in a shell:

Trying to create an instance of the AbstraceQueue returns an error message:

% AbstractQueue new
Cannot instantiate abstract class ::AbstractQueue

Create an instance of the concrete queue:

% set q [ListQueue new]

Enqueue and dequeue items

% $q enqueue 100
100
% $q enqueue 101
100 101
% $q dequeue
100
 
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