Chris Smith wrote:
apfelmus wrote:
For 1), it's enough to have a primitive
scheduleWriteTVar :: TVar a -> a -> STM ()
that ensures to write the TVar at the very end of the atomically block..
Unfortunately, though, this breaks the very thing that makes STM
attractive: namely, composability. Now in order to work with a TVar, I
need to know whether anything that came before me might have modified it,
and if so take the current value as a parameter instead of reading it
like normal.
Or am I misunderstanding something?
You're correct, that's what I meant. But it's nothing more and nothing
less than the purely functional way of dealing with "mutable" state,
isn't it?
And you need a parameter anyway, namely the TVar a itself. I mean,
when it's in scope like in
do
a <- readTVar v
writeTVar v (a+1)
readTVar v
you don't need a parameter. But if the do-block is broken up into
functions, you need a parameter
foo v = do
a <- readTVar v
writeTVar v (a+1)
bar v
bar v = readTVar v
and you may as well supply its value instead of the reference v .
Regards,
apfelmus
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