G'day all.
On Mon, Jul 07, 2003 at 04:04:17PM +0100, Sarah Thompson wrote:
> > I would also need to implement efficient indexes into the data structure
> > -- flat searching will be too slow for non-trivial amounts of data. In
> > C++, I'd implement one or more external (probably STL-based) indexes
> > that point into the main data structure.
I replied:
> The direct Haskell equivalent is to use Refs; either IORefs or
> STRefs. (I'd personally use IORefs, but that's because I like
> having IO around.) Dereferencing an IORef is a constant-time
> operation, just like chasing a pointer in C.
I forgot to give an example. :-)
Suppose you want some kind of electronic circuit, as you suggested.
Abstractly, you want something like this:
data Node
= Node NodeState [Component]
data Component
= Resistor ResistorCharacteristics Node Node
| Transistor TransistorCharacteristics Node Node Node
| {- etc -}
You can make this indirect in introducing refs:
type NodeRef = IORef Node
type ComponentRef = IORef Component
data Node
= Node NodeState [ComponentRef]
data Component
= Resistor ResistorCharacteristics NodeRef NodeRef
| Transistor TransistorCharacteristics NodeRef NodeRef NodeRef
The data structures are mutable:
getNodeState :: NodeRef -> IO NodeState
getNodeState node
= do Node state _ <- readIORef node
return state
setNodeState :: NodeRef -> NodeState -> IO ()
setNodeState node state
= do modifyIORef (\Node _ cs -> Node state cs) node
and it's straightforward to construct an index into the middle
somewhere:
type NamedNodeIndex = FiniteMap String NodeRef
Cheers,
Andrew Bromage
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