====== {-# LANGUAGE FlexibleInstances #-} ======
~~DISCUSSION~~

  * allows to implement instances that are of form (T a1 ... an) where a1 ... an are particlar types (not type variables)
    * [Double] instead of [a]
  * implies the application of extension ''[[codesnippets:typesynonyminstances|TypeSynonymInstances]]'' 
  * example:
    * code:<code Haskell>
main :: IO ()
main = 
    do
        print x1
        print x2

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

type MySpeed = Double

instance MyClass [MySpeed] where
    fMy xA xB = xA ++ xB

data MyData a = C a
    deriving Show

type MySpeedData = (MyData MySpeed)

instance MyClass MySpeedData where
    fMy (C xA) (C xB) = C (xA + xB)

x1 :: [MySpeed]
x1 = fMy [1.3] [2.5, 3.7]

x2 :: MySpeedData
x2 = fMy (C 1.3) (C 2.5)
</code>
    * compiler errors:<code>
app\Main.hs:12:10: error:
    * Illegal instance declaration for `MyClass [MySpeed]'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    * In the instance declaration for `MyClass [MySpeed]'
   |
12 | instance MyClass [MySpeed] where
   |          ^^^^^^^^^^^^^^^^^

app\Main.hs:20:10: error:
    * Illegal instance declaration for `MyClass MySpeedData'
        (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 MySpeedData'
   |
20 | instance MyClass MySpeedData where
   |          ^^^^^^^^^^^^^^^^^^^
</code>
  * example:
    * code:<code Haskell>
{-# LANGUAGE TypeSynonymInstances #-}

main :: IO ()
main = 
    do
        print x1
        print x2

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

type MySpeed = Double

instance MyClass [MySpeed] where
    fMy xA xB = xA ++ xB

data MyData a = C a
    deriving Show

type MySpeedData = (MyData MySpeed)

instance MyClass MySpeedData where
    fMy (C xA) (C xB) = C (xA + xB)

x1 :: [MySpeed]
x1 = fMy [1.3] [2.5, 3.7]

x2 :: MySpeedData
x2 = fMy (C 1.3) (C 2.5)
</code>
    * compiler errors:<code>
app\Main.hs:14:10: error:
    * Illegal instance declaration for `MyClass [MySpeed]'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    * In the instance declaration for `MyClass [MySpeed]'
   |
14 | instance MyClass [MySpeed] where
   |          ^^^^^^^^^^^^^^^^^

app\Main.hs:22:10: error:
    * Illegal instance declaration for `MyClass MySpeedData'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    * In the instance declaration for `MyClass MySpeedData'
   |
22 | instance MyClass MySpeedData where
   |          ^^^^^^^^^^^^^^^^^^^
</code>
  * example, with GHC option:
    * code:<code Haskell>
{-# LANGUAGE FlexibleInstances #-}

main :: IO ()
main = 
    do
        print x1
        print x2

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

type MySpeed = Double

instance MyClass [MySpeed] where
    fMy xA xB = xA ++ xB

data MyData a = C a
    deriving Show

type MySpeedData = (MyData MySpeed)

instance MyClass MySpeedData where
    fMy (C xA) (C xB) = C (xA + xB)

x1 :: [MySpeed]
x1 = fMy [1.3] [2.5, 3.7]

x2 :: MySpeedData
x2 = fMy (C 1.3) (C 2.5)
</code>
    * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
    * executes, with output:<code>
[1.3,2.5,3.7]
C 3.8
</code>


===== ✎ =====
~~DISCUSSION~~