Missing something with strictness

Question

I have this code:

divisors n = 1:[y|y<-[2..(n `div` 2)], n `mod` y == 0]

writeList l = do print "Start"
                 print l

Then, i want to call the function with strict argument; i tried:

writeList $! (divisors 12345678)

and

(divisors 12345678) \`seq\` (writeList (divisors 12345678))

But it does not behave as if it has strict argument: i.e. after the "Start" I have to wait for the evaluation of (divisors 12345678)

What am I missing?

Answer 1

seq(or $!) evalueates only for first head normal form, a (:) constructor in your case, you can use deepseq(or $!!) for evaluating to normal form.