Question

Here is the module – Number1.hs

module Number1(isTriangle) where

isTriangle x y z = if x*x+y*y >= z*z then True
               else False

This is the main program Main1.hs

import System.Environment
import Number1

main = do
    args<-getArgs
    let a = args !! 0
    let b = args !! 1
    let c = args !! 2
    if (IsTriangle a b c) then return(True) 
    else return(False)

This error I get when ghc --make Main1.hs

Answer 1

When you call isTriangle in Main1.hs, you call it with a capital ‘I’.

Make sure your capitalisation matches as Haskell is case sensitive, and make sure functions start with a lower case character as this is mandatory.

Edit – rounding up other errors

Main1.hs:

import System.Environment
import Number1

main :: IO()
main = do
         args<-getArgs
         {- Ideally you should check that there are at least 3 arguments
         before trying to read them, but that wasn't part of your
         question. -}
         let a = read (args !! 0) -- read converts [Char] to a number
         let b = read (args !! 1)
         let c = read (args !! 2)
         if (isTriangle a b c) then putStrLn "True"
           else putStrLn "False"

Number1.hs:

module Number1(isTriangle) where

{- It's always best to specify the type of a function so that both you and
   the compiler understand what you're trying to achieve.  It'll help you
   no end. -}
isTriangle       :: Int -> Int -> Int -> Bool
isTriangle x y z =  if x*x+y*y >= z*z then True
                      else False