Space leaks in Haskell

I have read it many times that lazy evaluation in Haske开发者_如何学Pythonll may sometimes lead to space leaks. What kind of code can lead to space leaks? How to detect them? And what precautions can be taken on part of a programmer to avoid them?

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You will get probably many answeres, this is the one, I have encountered when trying to do some 'real-world' application. I was using multithreading and some MVars to pass data around (MVar is something like locked shared memory). My typical pattern was:

a <- takeMVar mvar
putMVar mvar (a + 1)

And then, just sometimes, when a proper condition happened I did something like:

a <- takeMVar mvar
when (a > 10) ....

The problem is that the content of mvar was essentially (0 + 1 + 1 + 1 + ....)...which was quite intensive for numbers like 100k... This type of problem was quite pervasive in my code; unfortunately for multithreading applications it's very easy to get into such problems.

Detecting...what I did was starting haskell in the mode that produces data regarding memory consumption, starting and stopping different threads and looking if memory footprint is stable or not...

Anotomy of a thunk leak (with instructions how to debug it)

An example: Thunk memory leak as a result of map function

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I've run into this problem when doing recursion over large data structures. The built up thunks get to be too much and then you get a space leak.

In Haskell, you need to be constantly aware of the possibility of running into a space leak. Since iteration doesn't exist, basically any recursive function has the potential to generate a space leak.

To avoid this problem, memoize recursive functions, or rewrite them tail-recursively.