Where prelude alternatives/ classes and types to get values?
marco-oweber at gmx.de
Sat Feb 17 11:48:15 EST 2007
The prelude has some issues such like
It would be much nicer to use
class Empty where
empty :: a
except of more complicated error messages (especially for newbies)
Are there already some proposals?
Is this the right place to discuss these topics?
I got this idea because I'm currently learning more about webfunctions.
Some states (eg counters) are implemented using IORefs.
I could imagine a class
class ModifyIO m c where
modify :: IORef a -> ( a -> a ) -> m c
data Old a
data New a
data OldNew a = OldNew a a;
modify f = do a <- readIORef io
let new = f a
writeIORef io new
return $ f old new
instance ModifyIO IO (Old a) =
modify io f = modify io (\old _ -> Old old)
instance ModifyIO IO (New a) =
modify io f = modify io (\_ new -> New new)
instance ModifyIO IO (OldNew a) =
modify io f = modify io OldNew
which would lead to code such as
... = do (New count) <- modify ioCounterRef (+1)
... = do count <- readIORef ioCounterRef
let count = count + 1
writeIORef ioCounterRef count
Do you think this style is insane?
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