[Haskell-beginners] Confused by GLFW and more

Daniel Fischer daniel.is.fischer at googlemail.com
Thu Mar 3 18:13:35 CET 2011

On Thursday 03 March 2011 17:40:52, Britt Anderson wrote:
> This does fix the problem. Thank you. But raises a new question. Since
> that list was being constructed like this
>  where trialList oldps =
>           [[(h' i) , (p' i)] | i <- [1 ..], i < tn]

where trialList oldps =
    [[h' i, p' i] | i <- takeWhile (< tn) [1 .. ]]

The probem is that the compiler can't know that i never gets smaller than 
tn again once the first i >= tn is reached¹, so it is busy generating ever 
larger integers and testing them against tn, never finishing (well, it will 
finish when i is large enough to blow your memory, perhaps when
 i >= 2^(2^31) on a 32-bit system, I don't know how overflow of Integer is 

¹ An instance of Enum and Ord may wrap around, although that would be 
against the spirit of those two classes.

> it should have only had two elements when tn = 3. So, I assume that I
> am getting hit by a laziness issue, but how is it that I should have
> known that using mapM_ wouldn't have force thelist to exist in a
> finite form?

Laziness only in so far as it's necessary for [1 .. ]. The issue has 
nothing to do with monads or mapM_, it's the fact that the compiler doesn't 
use 'rules that should hold for instances of ...' and hence has to try on 
after a human knows it's over.

> By the way, I just now use "take (tn -1)" and all is good, but I would
> like to understand this issue better so I can avoid similar pitfalls
> in the future.

A similar pit is often fallen into with filter.

> Thx, Britt

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