I am trying to learn some Haskell by doing basic tasks in the particular case I am trying to implement some basic function for primality check, but I really can figure out the types, my code is
isPrime :: (Num a) => a -> Bool
isPrime n
| n <= 1 = False
| otherwise = not $ 0 `elem` map (mod n) [2..m] where m = floor $ sqrt n
I tried instead of (Num a) => a to use different number types or to use sqrt with fromIntegral but I still get error messages, like:
*Could not deduce (Floating a) arising from a use of `sqrt'
from the context: Num a
bound by the type signature for:
isPrime :: forall a. Num a => a -> Bool
at Helpers.hs:5:1-31
Possible fix:
add (Floating a) to the context of
the type signature for:
isPrime :: forall a. Num a => a -> Bool
* In the second argument of `($)', namely `sqrt n'
In the expression: floor $ sqrt n
In an equation for `m': m = floor $ sqrt n
| otherwise = not $ 0 `elem` map (mod n) [2..m] where m = floor $ sqrt n
I can really use some help here, thank you in advance.
You have two problems in your code:
Calling both sqrt n and (mod n) reqiure n to be both Floating and Integral at the same time.
(Num a) does not allow neither of the operations.A possible fix would be: a) narrow down type context to a much more concise (Integral a); b) add fromIntegral to sqrt's argument:
isPrime :: Integral a => a -> Bool
isPrime n
| n <= 1 = False
| otherwise = not $ 0 `elem` map (mod n) [2..m] where m = floor $ sqrt $fromIntegral n