[Haskell-cafe] How to write fast for loops

John Lato jwlato at gmail.com
Sun Apr 27 23:22:26 UTC 2014

On Sun, Apr 27, 2014 at 3:37 PM, Niklas Hambüchen <mail at nh2.me> wrote:

> On 27/04/14 23:18, John Lato wrote:> Hmm.  Are you using regular vectors
> or unboxed? Also what sort of crash?
> > Is it a stack space overflow or are you on a 32-bit platform?
> I'm using regular Data.Vector as V.

Are unboxed vectors faster?  My rule of thumb is to use them over
Data.Vector whenever possible.

> It's just eating my 8GB and then I kill it (or let my ulimit do it). It
> can't be the stack space since I'm on 7.6 where stack space is limited
> by default.
> It's on 64 bit Linux.

I would expect it's because you never force the argument.  With
`enumFromTo` the argument is forced because it needs to be checked for
termination, but `enumFromN` is probably building up a big chain of thunks.
 I guess for this case `enumFromN` has no benefit over `enumFromTo` because
the intention is to create a single loop instead of actually allocating the
vector, so the warning in the documentation doesn't necessarily apply.
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