No instance for (Integral Double) arising from a use of ‘floor’

user_12110750 0
Viewed 526

I'm trying to make a Haskell function that calculates x mod y:

fmod :: Double -> Double -> Double
fmod x y = x - y*m
    where m = floor a
          a = x / y

Running this in stack yields:

No instance for (Integral Double) arising from a use of `floor'

haskell
1 Answers

The return type of a floor :: (RealFrac a, Integral b) => a -> b is an Integral type, a Double is not an Integral type, so the m in your code can not be a Double, this is important, since the (*) :: Num a => a -> a -> a and (-) :: Num a => a -> a -> a functions return the same type as both operands.

You can however make use of fromInteger :: Num a => Integer -> a to convert an Integer to any Num type. So you can work with:

fmod :: Double -> Double -> Double
fmod x y = x - y * fromInteger (floor (x / y))
user_675790
Related