Because mod :: Integral a => a -> a -> a and sqrt :: Floating a => a -> a,
the final type of primes' ends up being :: (Floating a, Integral a) => [a],
which doesn't work very well obviously.
The easiest solution is to try
let primes' = 2 : [p | p <- odds, null [d | d <- (takeWhile (<=ceiling
(sqrt (fromInteger p))) odds), p `mod` d == 0]]
You can make your own sqrt if you want to clean it up.
let isqrt = ceiling . sqrt . fromInteger
On Fri, Jul 1, 2016 at 9:41 PM, Russ Abbott
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.
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