NB. The stack object template which inherits a print method

data =: ''

stack_z_ =: monad def 0
'method value' =. 2 {. y.
if. method -: 'type'
    do. 'stack'
  elseif. method -: 'emptyp'
    do. 0 = # data
  elseif. method -: 'push'
    do. for_effect_only data =: (< value) , data
  elseif. method -: 'top'
    do. if. 0 = # data
          do. 'top:  stack is empty'
          else. > {. data
        end.
  elseif. method -: 'pop'
    do. if. 0 = # data
          do. 'pop:  stack is empty'
          else. for_effect_only data =: }. data
        end.
  elseif. method -: 'size'
    do. # data
  elseif. method -: 'print'
    do. if. 0 = # data
          do. 'print: stack is empty'
          else. for_effect_only display 'top of stack'
                print 'print' ; < data
        end.
  elseif. 1
    do. root_object method ; value
end.
)