====== {-# OPTIONS_GHC -fno-warn-orphans #-} ======
~~DISCUSSION~~

  * 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 [[https://wiki.haskell.org/Orphan_instance|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: [[codesnippets:datastructuresfromsymboltrees|Creating data structures from symbol trees]]
  * example:
    * code of module Lib:<code Haskell>
module Lib
    (
        MyClass(..)
    ) where

class Num x => MyClass x where
    fMy :: x -> x
</code>
    * code of module Main:<code Haskell>
import Lib (MyClass(..))

main :: IO ()
main = print x1

instance MyClass Double where
    fMy x = (x * x) + x + 1

x1 :: Double
x1 = fMy 3.4 
</code>
    * compiler warning, when using -Wall:<code>
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
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
</code>
  * example, with GHC option:
    * code:<code Haskell>
{-# 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 
</code>
    * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
    * executes, with output:<code>
15.959999999999999
</code>


===== ✎ =====
~~DISCUSSION~~