[Haskell-cafe] relational data representation in memory using haskell?

[Marc Weber]

Hi Salvatore

On Thu, May 22, 2008 at 11:01:01AM +0200, Salvatore Insalaco wrote:
> > Consider
> >
> > let x = Cd ...
> > forkIO $ ( do something with x } -- (1)
> > print x -- (2)
> >
> > How can ghc know when running line (2) that (1) hasen't changed the
> > record? I see two solutions:
> > a) give the forked process a copy (Then my design will collapse)
> >   but this is expensive to copy data without knowing you ned to
> > b) use pointers and replace x ony on updating. Thus if (1) changes the
> >   title a new struct wil be created poiting to the old list but a new
> >   title String. line (2) doesn't have to care at all.
> 
> GHC knows that because in Haskell isn't possible to "update" x. x is
> not a variable, it's a binding.
> To put it simply: with IORefs (and STRefs, MVars, ...) you have
> references to values that you can change (inside their respective
> monads), much like variables, but data declarations are values, not
> references to values (even if GHC stores them as pointers you cannot
> treat them as such), so you cannot update it.
Sorry - maybe I'm unable to express using the correct terminology..
So I'll just give a small example how I think it could magically work?

  data CD = CD { title :: String, tracks :: [ Track ] }
  data Track = Track { track :: String, cd :: CD }
  data PDB = PDB { cds :: Set CD, tracks :: Set Track }

Let's fill the database with 1 track and a cd:

0x3 = pointer to DB rec
0x1: adress of CD
0x5: adress of Track
0x4, 0x9, 0x9: start adress of linked list connected by pointers..
    In the final solution should use finger trees or such to speed up
    deletion / replacing elements

0x3 database:
        0x8 cds :
                tuple1 0x1 : (0x6 "My song")     (0x4 [ 0x5, ... ])
                              ^ pointer to str    ^ pointer to track list, 0x5 = pointer to track

        0x9 tracks:
                tuple1 0x5 : ( 0x7 "track 1") 0x1 
                                        ^ reference to cd
        
Now I query the track, and "update" it (replacing the title)..
It's a little bit tricky, because when updating the track I need to
update the cd as well (circular referency). All new pointers are
starting from 0x20

So in haskell it would look like this:
        let updatedCd = 0x22  CD (0x6 "My song")  (0x20 ( 0x23 : ...)
            updatedTrack = 0x23 Track ( 0x21 "updated track title" ) 0x22
        in (0x27) DB (0x24 (updatedCd:otherCds)) (0x25 (updatedTrack:otherTracks))

Now my new address to access the database is 0x25. So pretty every
adress has been changed but 0x6, ..., otherCds and otherTracks
A query running using db 0x3 will not notice any change on its snapshot.
Are these actions called rebinding?

Of course if you have a lot of relations writing this
        let 
        in
will become tedious and error prone.. That's why I'd like to use
template haskell to automatically derive it.

Thanks for listening

Marc Weber