I'm trying to define primes using just list comprehension. The following works.

Prelude> odds = [3, 5..]

Prelude> primes = 2 : [p | p <- odds, null [d | d <- take (p `div` 4) odds, p `mod` d == 0]]

But when I replace "take (p `div` 4)" with takeWhile (<=(sqrt p)) as in

Prelude> primes' = 2 : [p | p <- odds, null [d | d <- (takeWhile (<=(sqrt p)) odds), p `mod` d == 0]]

I get no error message on entering the statement, but when I run

Prelude> take 8 primes'

I get an error message that I can't understand.

<interactive>:23:1: error:

• Ambiguous type variable ‘a0’ arising from a use of ‘it’

prevents the constraint ‘(Floating a0)’ from being solved.

Probable fix: use a type annotation to specify what ‘a0’ should be.

These potential instances exist:

instance Floating Double -- Defined in ‘GHC.Float’

instance Floating Float -- Defined in ‘GHC.Float’

• In the first argument of ‘print’, namely ‘it’

In a stmt of an interactive GHCi command: print it

I tried using (ceiling (sqrt p)), and I tried to explicitly declare it an Int, but neither of those helped.

I'd appreciate some help.

Thanks.

P.S. The following runs fine.

Prelude> takeWhile (<= (sqrt 99)) odds

So I'm completely stumped.