{-# LANGUAGE FlexibleInstances #-}

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 TypeSynonymInstances

example:

code:

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)

compiler errors:

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
   |          ^^^^^^^^^^^^^^^^^^^

example:

code:

{-# 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)

compiler errors:

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
   |          ^^^^^^^^^^^^^^^^^^^

example, with GHC option:

code:

{-# 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)

compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
executes, with output:
```
[1.3,2.5,3.7]
C 3.8
```

✎

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