[Haskell-beginners] Applicability of FP unchanging things to real world changing things

Glurk streborg at hotmail.com
Sat Dec 12 21:11:28 EST 2009

I come from an OO background, but have recently started to see the light and 
think that functional programming has a lot to offer in real world, general 
software development - not just some small specific niche.

Having the OO background certainly has limited the way I view software 
development, and I guess in learning something new, I have a tendency to 
compare it with what I do know, looking for similarities and differences.

So, perhaps that's why I am having difficulty in seeing how some functional 
programming ideas apply.

What's got me a little puzzled at the moment is non mutable data.
On the one hand, I think it's great that I can mostly forget about errors 
caused by my variables having unexpected values, and that I can reason about 
my program much more easily...

However, on the other hand, I wonder how it ties in with real world systems 
that do in fact do deal with objects that change state.

For example, if I was building a library system - I might have book objects, 
and if a book gets borrowed, there remains the same book object, but it now 
just has a different state - from "on shelf" say, to "borrowed".

Now, in Haskell, I might have a function to lend a book :-

lend book1

and this is going to return a new book, with the appropriate status change..
..but what of the old book ? I don't want 2 book objects floating around..
...to my way of thinking, it's really not a new book, it really is a change of 

How do people see this kind of situation ?
Is it just the way of looking at it... ?
Do I just accept that I toss out my old library object, and have a new one, 
which happens to be the same as the old one, except for the "new" book which 
is now borrowed ?

Do I really need to get over thinking of objects in the first place, even 
though it's what I see exist in the real world ?

Is there where we part company with non mutability and venture into the monads 
where we do have side effects ?

Any thoughts appreciated ! :)

Thanks !

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