Tomasz Zielonka wrote:
On Tue, Nov 23, 2004 at 08:50:45PM -0800, John Meacham wrote:
Atom.hs from ginsu..
This is perhaps the best example, and an incredibly useful piece of code
for anyone struggling with space problems out there.
it provides
data Atom = ... (abstract)
instance Ord Atom
instance Eq Atom
toAtom :: String -> Atom
fromAtom :: Atom -> String
[...]
internally, Atom has a global hash table of strings -> atoms, note that
externally, Atom is truly purely functional. toAtom and fromAtom
although using internal state inside are real functions. the same
argument always returns the same (externally visible) result. This is
because the actual integer chosen is hidden, there is no way to get at
it outside the module.
Just a nitpick: will this code always yield the same results?
map fromAtom $ sort $ map toAtom $ words "Just a nitpick"
This is one of the very few cases where I've used unsafePerformIO
(because I too have implemented something like Atom :). To make
this work properly you need to actually compare the strings for
Ord, but you can compare "pointers" for Eq. Doing that, you don't
break Haskell semantics (but proving it seems tricky).
-- Lennart
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