{-# OPTIONS_GHC -fno-warn-orphans #-}

prevents warnings of orphan instance, meaning:
- instance declaration of a type is not declared in the module of the class
NOTE: Orphan instances may be useful.
- see also Haskell wiki - Orphan instance
NOTE: The purpose of the examples here is only to demonstrate the impact on the compiler. The examples may not make sense as a use case of type classes.
- example that may make more sense: Creating data structures from symbol trees

example:

code of module Lib:

module Lib
    (
        MyClass(..)
    ) where
 
class Num x => MyClass x where
    fMy :: x -> x

code of module Main:

import Lib (MyClass(..))
 
main :: IO ()
main = print x1
 
instance MyClass Double where
    fMy x = (x * x) + x + 1
 
x1 :: Double
x1 = fMy 3.4

compiler warning, when using -Wall:

app\Main.hs:6:1: warning: [-Worphans]
    Orphan instance: instance MyClass Double
    To avoid this
        move the instance declaration to the module of the class or of the type, or
        wrap the type with a newtype and declare the instance on the new type.
  |
6 | instance MyClass Double where
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

example, with GHC option:

code:

{-# OPTIONS_GHC -fno-warn-orphans #-}
 
import Lib (MyClass(..))
 
main :: IO ()
main = print x1
 
instance MyClass Double where
    fMy x = (x * x) + x + 1
 
x1 :: Double
x1 = fMy 3.4

compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
executes, with output:
```
15.959999999999999
```

{-# OPTIONS_GHC -fno-warn-orphans #-}

✎