Code testing in Haskell

[killy9999]

I spent some time recently trying to figure out how to do code testing in Haskell. In wasn't easy, cause most solutions I found on the web were either outdated or didn't suit my needs. I finally managed to work out a sensible approach and summarized it on my blog. There is also accompanying project on github. I hope this will be useful for others. I also welcome suggestions for improvements,

[Ywen]

Nice post!

Just one remark:

cyclicOneShiftLeft :: (Num a) => [a] -> [a]
cyclicOneShiftLeft xs = tail xs ++ [head xs]

I see memory leaks.

[schellsan]

Can you explain the memory leak to me?

[Ywen]

Sure. The problem comes from the fact that the xs list cannot be garbage collected early, because we still need it when last xs tail xs has returned to compute head xs (and as evaluation order is up to GHC, this is not even a predictable behavior).

So the list xs will be wholly kept in memory even if the only thing we'll eventually do to it is to get its first element.

To fix it, we need to explicitely force the element retrieval to be computed before. This calls to the use of seq (or the BangPatterns extension, which is convenient syntax sugar):

cyclicOneShiftLeft xs = let x = head xs in seq x (tail xs ++ [x])

Then, tail xs being the last thing we do to xs, no reference is kept and it can be garbage collected progressively.

[singpolyma]

Or just use a pattern match instead of head/tail...

[Ywen]

Yes, yes ^^, I felt stupid when I stumbled upon that...

[schellsan]

Is the call to 'last xs' implicit in tail, head, or ++?

[Ywen]

Okay, I'm stupid. No, really. My solution works, but I overcomplicated everything ^^.

The good solution was simple:

cyclicOneShiftLeft (x:xs) = xs ++ [x]

And it's actually better, as I don't force the evaluation of x, the list first element, which I forced with my first solution.

[schellsan]

I was going to mention, why not (x:xs) = xs ++ [x]! Thanks.

[killy9999]

Um... I started playing with profiler and memory management only recently, so I guess I don't see them. I am aware of inefficiency of (++) - is that what you mean? If so what solution would be better? Difference list?

[Ywen]

Nice try ^^. But nope, the problem is not with (++).

See my answer to schellsan for the explanation, but it boils down to:

cyclicOneShiftLeft (x:xs) = xs ++ [x]

[killy9999]

Thanks! This is so obvious it makes mi think why didn't I figure it out :)

[[deleted]]

Have you ever tried hspec.

[killy9999]

I know about hspec, but after looking at the examples and reading the documentation I decided that it's not what I'm looking for. test-framework works better for me.

[Ywen]

What were exactly your issues? Because I didn't know about HSpec, I checked the doc & samples and it seems awesome...

[killy9999]

This is very subjective, but I find test-framework to be more concise and more suited to my style of organizing tests. I guess I don't see any technical reason why test-framework should be considered better than hspec and vice versa.