====== {-# LANGUAGE TypeSynonymInstances #-} ======

  * allows to use type synonyms as instances
  * example:
    * code:<code Haskell>
main :: IO ()
main = print x1

class MyClass x where
    fMy :: x -> x -> x

type Speed = Double

instance MyClass Speed where
    fMy xA xB = xA + xB

x1 :: Speed
x1 = fMy 1.3 2.5
</code>
    * compiler error:<code>
app\Main.hs:9:10: error:
    * Illegal instance declaration for `MyClass Speed'
        (All instance types must be of the form (T t1 ... tn)
         where T is not a synonym.
         Use TypeSynonymInstances if you want to disable this.)
    * In the instance declaration for `MyClass Speed'
  |
9 | instance MyClass Speed where
  |          ^^^^^^^^^^^^^
</code>
  * example, with GHC option:
    * code:<code Haskell>
{-# LANGUAGE TypeSynonymInstances #-}

main :: IO ()
main = print x1

class MyClass x where
    fMy :: x -> x -> x

type Speed = Double

instance MyClass Speed where
    fMy xA xB = xA + xB

x1 :: Speed
x1 = fMy 1.3 2.5
</code>
    * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
    * executes, with output:<code>
3.8
</code>


===== ✎ =====
~~DISCUSSION~~