I have just uploaded a new version of
ghc-heap-view to Hackage that provides Evaluation state assertions in the module
GHC.AssertNF.
Imagine you are writing a web application in Haskell that sports a global number-of-visitors counter in an
IORef Int. For every request, you call
modifyIORef (+1). Eventually, you notice your very popular web site to hog more and more memory. So you browse to the internal page that shows the counter, and you have to wait for a long time until you eventually see the result (or get a stack overflow). The reason: The applications of
(+1) were not performed until you looked at the number; instead, a long chain of such computation first filled your heap and then your stack.
So you have learned the hard way that you might want to avoid space leaks, and want calculations to be done during the request that caused them, and want the
IORef to always contain fully evaluated data. So you stumble about
modifyIORef' in
Data.IORef and indeed, this fixes your problem.
Later, you notice that you want to count
POST and
GET requests separately. You change the type to
IORef (Int, Int) and call
modifyIORef' (first (+1)) or
modifyIORef' (second (+1)). And suddenly, the space leak is back (which you only notice after the next push to the real site, because your local tests never caused enough requests to make it noticeable). So you not only want to fix it, you also want to ensure that it does not break again.
In other words, you want to ensure the policy that values stored in an
IORef are always in normal form. You achieve this with the following alternative to
modifyIORef':
modifyIORef'Assert :: IORef a -> (a -> a) -> IO ()
modifyIORef'Assert ref f = do
x <- readIORef ref
let x' = f x
x' seq return ()
assertNF x'
writeIORef ref x'
Using this instead of
modifyIORef' will print this warning to standard error output right the first time you call
modifyIORef'Assert (first (+1)):
Parameter not in normal form: 2 thunks found:
let x1 = (S# 1,S# 1)
in _bh (_thunk x1 (_bco (S# 1)),_sel x1)
(Otherwise, the program runs as usual.) So obviously, you need to use a strict variant of
first (or
strict pairs):
first' :: (a -> b) -> (a, c) -> (b, c)
first' f (x,y) = let x' = f x; r = (x', y) in x' seq r seq r
With this, the warning goes away. Whenever you now change the type of the
IORef or modify it in a too-lazy-way, you can be sure that you ll be warned about it, before the space leak itself becomes noticeable.
In the production code, you might want to disable the check. For that, simply put
disableAssertNF somewhere in your main function.
Why is this better than just calling
deepseq in
modifyIORef'Assert? Because this way, the code still creates unwanted thunks that are then evaluated before storing them in the
IORef, whereas with
assertNF you are told about the thunks and can prevent them from being created in the first place. Also,
assertNF does not add a type class constraint.
This is just one example application for
assertNF (and its variants
assertNFNamed, which includes a name in the warning to better spot the cause, and
$assertNFHere, which uses Template Haskell to include the current source code position in the warning), and I hope that there are more. If you happen to make use of it, I d like to hear your story.
Interesting stuff is happening again and I m doing a bit of travelling where I m not driving much, so I can write some blog posts. If this train stops bouncing quite so much!
I think most readers are interested in technology and collaborative work, so it makes sense to alternate those two themes most of the time. So that s what I ll aim for, probably a few posts each week for the next few weeks.
Let me know in the comments or our co-op s contract form if there s anything in particular you d like us to cover, else I ll start with my recent experiences installing Debian 6 on a new laptop and the fun of running a business at tax return time.
Update: I apologise for spamming all the nice people on Planet Debian with this unrelated post in foreign language it got mis-tagged and (although I tried classifying it correctly and removing it from the separate Debian-only RSS feed going to the Debian Planet) Planet software just keeps it around until admins get a spare moment to remove it manually. The basic idea of the post is that there is a e-government service by the Latvian government that provides any citizen a way to check what their pension would be, but it is kinda useless, because it calculates what you pension would be if you decided to retire right now. So in the post below I provide a few simple steps on how to estimate what the real pension could be (in todays currency values) if a person kept paying into the system at the same rate until the normal retirement age.
Latvija.lv ir 
It s been a long time I wanted to blog about my recent work on the Openstack packaging. I finally can find a bit of time to do it.
For those who don t know yet 
Next round of 
Some 
This post is a quick status update on 
Since some people asked for it, and since it's (unfortunately) fairly
nontrivial, here is the script that I'm using currently to transcode a
.dv file into webm: