The write*Ref functions, like many "write into data structure" have the common
problem of being much lazier than you want. The nextState calls form a lazy
thunk. In fact it tries form 10^6 nested thunks to call nextState. So you have
to use something like seq to reduce the laziness:
step :: Tag s -> ST s (Maybe Integer)
step t = do
c <- readSTRef (count t)
s <- readSTRef (state t)
writeSTRef (count t) (c - 1)
let state'=nextState s
state' `seq` writeSTRef (state t) state'
if (c <= 0) then return Nothing else return (Just c)
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