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--- codesnippets:flexibleinstances [2021/04/12 18:33] – created f2b216
+++ codesnippets:flexibleinstances [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== {-# 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]'
+   |
+| 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'
+   |
+| 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]'
+   |
+| 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'
+   |
+| 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~~