Multiple "pointers" to "objects"

Ron de Bruijn
Sat, 12 Jul 2003 07:47:02 -0700 (PDT)

Hi there,

I was almost certain that Haskell was a great language
until I wanted to make a real usefull program and got
the following problem.

I have
data Lesson = Lesson Teacher SomeOtherProperties
     deriving Show

data Subject = Subject Name [Teacher]
     deriving Show

data Teacher = Teacher TimeTable SomeOtherProperties

What I want is that when I put some lesson in my
timetable, the resources needed for that lesson are
used up, so for example the timetable of a teacher
will fill with each lesson that it gives. 

The problem is that when I "model" it this way, the
state of the teachers that can give a certain subject
will not change (suppose I have some function that
fills one timeslot of the timetable of a teacher).

It makes it even harder, because of the fact that one
teacher can teach multiple subjects.

In an OO-language I would simply let each element of
the list of TeacherObjects of Subjects point to some
TeacherObject, so it remembers it state, but that's

It's ofcourse possible to put a list of Subjects that
a Teacher teaches in the data declaration of the
teacher. But then there is no way of saying
efficiently (O(1) Just a pointer or index):"Give me a
list of all teachers that give Physics", and that's
just what I need.

I could use a hashtable which includes the
teachersobjects as values and the subjects as keys,
but that isn't a very beautiful solution. This would
give me(building of Hashtable O(n) and getting all
teachers of some subject O(1)), so it would do.  

I am almost sure there exist some nice way of doing
this, because otherwise Haskell would be completely
useless IMHO, but I don't know it.

Do you have any idea? 

Greets Ron

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