CODEWITHSUNDEEP
haskell
jackrabbithanna
jackrabbithanna

Reputation: 167

How to input two integers from command line and return square root of sum of squares

I'm trying to start learning Haskell. I want to enter two numbers from the command line and and return the sqrt root of the sum of the square of each number. This is the Pythagorean Theorem

Surely I thought I'd find an example somewhere, so I could wrap my head around taking some input, passing the input to a function, returning it, and printing the result out. Trying to work through this simple case. PHP / Javascript programmer, wanting to learn functional programming, so like I'm learning Martian at this point. Sorry if this question's been asked or is too simple. Surely I'm close, but I don't understand what I'm missing. I understand the sqrt would return a floating point number.

module Main where

hypotenuse a b = sqrt $ a * a + b * b
main :: IO ()
main = do
  input1 <- getLine
  input2 <- getLine
  let a = read input1 :: Int
  let b = read input2 :: Int
  print $ hypotenuse a b

This returns an error:

No instance for (Floating Int) arising from a use of ‘hypotenuse', line 10, character 11

the 'h' in hypotenuse is highlighted in my Atom editor IDE. with the ghc-mod plugin to check.

UPDATE: @peers answer solved my problem ...

Thanks stackoverflow.com, my first haskell program https://github.com/jackrabbithanna/haskell-pythagorean-theorem

Upvotes: 1

Views: 613

Answers (1)

peer
peer

Reputation: 4699

sqrt expects input of type class Floating but you provide Ints which do not instantiate Floating. In ghci you can see the type signature of sqrt with :t sqrt. It is sqrt :: Floating a => a -> a.
Int implements several type classes as can be seen with :info Int:

instance Eq Int -- Defined in ‘GHC.Classes’
instance Ord Int -- Defined in ‘GHC.Classes’
instance Show Int -- Defined in ‘GHC.Show’
instance Read Int -- Defined in ‘GHC.Read’
instance Enum Int -- Defined in ‘GHC.Enum’
instance Num Int -- Defined in ‘GHC.Num’
instance Real Int -- Defined in ‘GHC.Real’
instance Integral Int -- Defined in ‘GHC.Real’
instance Bounded Int -- Defined in ‘GHC.Enum’

but Floating is not among them.
Try reading as Double or converting the Ints with fromIntegral.

Both ways in the code:

module Main where

hypotenuse a b = sqrt $ a * a + b * b
main :: IO ()
main = do
  input1 <- getLine
  input2 <- getLine
  let a = read input1 :: Double
  let b = read input2 :: Int
  print $ hypotenuse a (fromIntegral b)

Upvotes: 5

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