Odd Performance

[Tom Pledger]

Tim Otten writes:
 :
 | Can anyone suggest why the tighter algorithm exhibits significantly
 | worse performance? Is takeWhile significicantly more expensive than
 | take?

No.

 | Is the \z lambda expression expensive?

No.

 | The intsqrt isn't recalculated each time takeWhile evalutes a
 | prime, is it?

Probably.  Try replacing this

    (\z -> z <= (intsqrt x))

with this

    (\z -> z^2 <= x)

or moving the O(sqrt(n)) step[1] into a let expression

    let zmax = intsqrt x in ... (\z -> z <= zmax) ...

 | Slightly confused,
 | Tim Otten
 | 
 | [1] Note that intsqrt calculates the floor of the square root of x.
 |     intsqrt x = intsqrt' 1 x
 |     intsqrt' n x
 |         | (n*n > x) = n-1
 |         | otherwise = intsqrt' (n+1) x