[Haskell-cafe] Re: I just don't get it (data structures and OO)

[Phlex]

apfelmus wrote:
> Phlex wrote:
>   
>> Donald Bruce Stewart wrote:
>>     
> Yes and no. The point is that if you can't automate it, you have to code
> it by hand anyway which constitutes most of the hairiness. But I know
> what you mean and there's a nice way to do that with multi-parameter
> type classes.
>
> Let's assume a data structure
>
>   data Universe = Universe [Galaxy]
>   data Galaxy   = Galaxy   [Planet]
>   data Planet   = Planet   { name :: String, size :: Double }
>
> The insight is that in order to reference a planet inside the universe
> structure, you have to know the path from the top. In other words, you
> have to know that's it's the 2nd planet from the 4th galaxy before you
> look up its name. If you don't throw that information away, you can use
> it to update it as well. In effect, the Universe behaves like a finite
> map with composite keys.
>
>   {-# OPTIONS_GHC -fglasgow-exts -#}
>   import Prelude hiding (lookup)
>
>   class Map map key a | map key -> a where
>       lookup ::             key -> map -> Maybe a
>       adjust :: (a -> a) -> key -> map -> map
>
>   instance Map [a] Int a where
>       lookup 0 [x]    = Just x
>       lookup 0 _      = Nothing
>       lookup k (x:xs) = lookup (k-1) xs
>       lookup _ _      = Nothing
>
>       adjust f 0 [x]    = [f x]
>       adjust _ 0 xs     = error "Index out of range"
>       adjust f k (x:xs) = x : adjust f (k-1) xs
>       adjust f _ xs     = error "Index out of range"
>
>   instance Map Universe Int Galaxy where
>       lookup k (Universe gs)   = lookup k gs
>       adjust f k (Universe gs) = Universe (adjust f k gs)
>
>   instance Map Galaxy Int Planet where
>       lookup k (Galaxy ps)   = lookup k ps
>       adjust f k (Galaxy ps) = Galaxy (adjust f k ps)
>
>   instance (Map m k m', Map m' k' a) => Map m (k,k') a where
>       lookup (k,k') m   = lookup k m >>= lookup k'
>       adjust f (k,k') m = adjust (adjust f k') k m
>
>
> You can lookup the 2nd planet in the 4th galaxy with
>
>   lookup (4,2) universe :: Maybe Planet
>
> and you can update it via
>
>   adjust (\planet -> planet {name = "Earth"}) (4,2) universe
>
> Thanks to type-classes and overloading, you can still access single
> galaxies with
>
>   lookup 4 universe :: Maybe Galaxy
>
> Regards,
> apfelmus

This answers precisely my question.
Hiding the access/update methods behind a common interface, that's the 
key ...

Thanks a lot,
Sacha