My apologies, my space leak was in my implementation of runAuto. Ignore me!
b
On Wed, Sep 1, 2010 at 3:01 PM, Ben
Hello Arrow-theorists --
In a previous email Maciej Piechotka showed me how to construct a recursive function I wanted via ArrowCircuits. This computes the running sum of a stream of Ints.
rSum :: ArrowCircuit a => a Int Int rSum = proc x -> do rec let next = out + x out <- delay 0 -< next returnA -< next
Although this arrow runs in linear time, it exhausts the stack (I've compiled with ghc -02.) The obvious strict non-arrow version runs in linear time and constant space :
rSum :: [Int] -> [Int] rSum = rSum' 0 where rSum' _ [] = [] rSum' n (x:xs) = let n' = x+n in n' `seq` n' : rSum n' xs
It is ironic because arrows were used in the past to plug space leaks. It seems that using delay here introduces a growing stack of thunks. I have tried decorating everything I could think of with `seq` in the pointfree and pointed versions, to no avail.
Is there a (pointed or point-free) version which runs in linear time and constant space?
Best regards, Ben
PS I tried manually translating the ArrowCircuit into a length-preserving stream function (SF in Hughes's paper) to try to understand what was going on. I ended up with
rSum :: [Int] -> [Int] rSum xs = zipWith (+) xs $ 0 : rSum xs
which is not quite right as it is quadratic. But I think it captures the basic idea of the translation.