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.