Ordering an array of numbers in Haskell-CodePudding

I am trying to use generic programming in Haskell and need to sort an array of numbers but for some reason when I run the code, I receive an error stating "No instance for (Fractional Nums) In the expression: 645.41...." Every time I look at my code, I think it makes sense, but I'm not sure why it does not work...

import Data.List (sortBy)
import Data.Ord (comparing)
    
    
data Nums = Nums {numbers::Double} deriving(Ord, Eq, Show)
sortNums :: [Nums] -> [Nums]
sortNums = sortBy(comparing numbers)
arr =  [645.41, 37.59, 76.41, 5.31, 1.11, 1.10, 23.46, 635.47, 467.83, 62.25]
main:: IO ()

     main = 
          do
               print(sortNums arr)

I apologize if this code looks messy or does not make sense, I am new to Haskell....

CodePudding user response：

Unless you have a good reason that's not obvious from the question, you should probably just delete Nums entirely.

import Data.List (sort)

arr :: [Double]
arr = [645.41, 37.59, 76.41, 5.31, 1.11, 1.10, 23.46, 635.47, 467.83, 62.25]

main :: IO ()
main = print (sort arr)

CodePudding user response：

The problem here is that arr is an array of Doubles, but you are calling sortNums on it, which requires an array of Nums. While these are effectively "the same" type, Haskell doesn't do any automatic conversions for you. If you replace print(sortNums arr) with print (sortNums (map Nums arr)) - to ensure the list you're sorting has each member a Nums value - then this works as intended.

(The error message you received here is unfortunately rather confusing and unhelpful, but derives from the fact that literal floating-point values like 645.41 can represent a value of any type that's an instance of the Fractional typeclass. GHC sees you're trying to apply sortNums to the list, and that this needs a list of Nums, so it tries to make the values in the list of type Nums - which it can do, but only if there is a Fractional instance, which is why you get that specific error message. Don't worry too much about this - I'm just trying to explain why you get this particular error rather than one which would be more helpful like "couldn't match type [Double] with expected type [Nums]")