When advocating functional languages like Haskell, one of the claims
I've tended to make is that referential transparency allows the
language to be much more aggressive about things like common
subexpression elimination (CSE) than traditional imperative languages
(which need to worry about preserving proper side-effect sequencing).
But a recent example has left me thinking that maybe I've gone too
far in my claims.
First, lets consider a simple consumer program, such as:
printEveryNth c l n = do print (c', x)
printEveryNth c' xs n
where c' = c+n
x:xs = drop (n-1) l
Note that we can pass this function an infinite list, such as [1..],
and it won't retain the whole list as it prints out every nth element
of the list.
Now let's consider two possible infinite lists we might pass to our
consumer function. We'll use a list of primes (inspired by the
recent discussion of primes, but you can ignore the exact function
being computed). Here's the first version:
primes = 2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0)
(factorsToTry x)]
where
factorsToTry x = takeWhile (\p -> p*p <= x) primes
As you might expect, at the point where we print the nth prime from
our infinite list, we will be retaining a list that requires O(n) space.
But this simple modification allows us to use only O(sqrt(n)) space
at the point we print the nth prime:
primes =
2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0) (factorsToTry x)]
where
slowerPrimes =
2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0)
(factorsToTry x)]
factorsToTry x = takeWhile (\p -> p*p <= x) slowerPrimes
Notice the gigantic common subexpression -- both primes and
slowerPrimes define exactly the same list, but at the point where
we're examining the nth element of primes, we'll only have advanced
to the sqrt(n)th element of slowerPrimes.
Clearly, "simplifying" the second version of primes into the first by
performing CSE actually makes the code much *worse*. This "CSE-
makes-it-worse" property strikes me as "interesting".
So, is it "interesting"...? Has anyone worked on characterizing CSE
space leaks (and avoiding CSE in those cases)? FWIW, it looks like
others have run into the same problem, since bug #947 in GHC (from
October 2006) seems to be along similar lines.
Melissa.
P.S. These issues do make massive difference in practice. There is
a huge difference between taking O(n) and O(sqrt(n)) space -- the
difference between a couple of megabytes for the heap and tens or
hundreds of megabytes.
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