[Unicon-group] Tax Payer problem

[Majorinc, Kazimir]

[I have send this on icon-group, but I didn't seen it appeared, so I'm 
sending it here. Sorry if you seen it twice. ]

Few years ago, I noticed that this simple Tax payer problem is surprisingly 
hard for OO programming. Actually, because of that problem I finally 
concluded that OO is too much complication for too little power and gave up 
from it.

Icon solution is, however, very elegant.
==================
1) Write program that calculates tax for different persons.
2) Each person is member of some group (soldier, professor ... ), and each 
group  has own rules for tax calculations. For example, tax for soldiers is 
salary/10.

3) For addition of each new group (and tax rule) one should write only one 
new function (or class), somewhere in the code. Existing code should not be 
changed!

Until now, it was easy but ...

4) One person can be member of a few groups in the same time (and 
membership can change during run time)

5) In that case, tax is maximum of all taxes calculated on the basis of 
groups membership.

====================================

# GENERAL PART, UNNECESSARY BUT MAKES SPECIAL PART PRETTIER

procedure poly(s,X)

  # returns list [proc(s||t1)(X),...,proc(s||tn)(X)]
  # where t1,...,tn are elements of X.member_of
  result:=[]
  every ti:=!X.member_of do
    put(result, proc(s||"_"||ti)(X)) # <= most important line
  return result
  end
procedure max(L)

  if *L > 0 then
    {
      result:=L[1]
      every result <:= !L
      return result
    }
  end
# SPECIAL PART

invocable all
record person(salary, member_of) #member_of is special!
procedure tax_soldier(X)
  return X.salary/10.0
  end
procedure tax_professor(X)
  return X.salary/20.0 + 100
  end
procedure tax(X)
  return max(poly("tax",X)) # :^)
  end
procedure main()
  A:=person(1000.0, [ "soldier" ])
  B:=person(1000.0, [ "professor" ])
  C:=person(1000.0, [ "soldier", "professor" ])
  write(tax(A) ,", ", tax(B) ,", ", tax(C)) # this line is goal
  end
# THATS ALL.

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Kazimir Majorinc, Zagreb, Croatia