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--- codesnippets:typesynonyminstances [2021/04/12 19:14] – created f2b216
+++ codesnippets:typesynonyminstances [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== {-# 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'
+  |
+| 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>
+.8
+</code>
+===== ✎ =====
+~~DISCUSSION~~