Document GHC-93557

Author: David Binder

message-index/messages/GHC-93557/illegalSynonymInstance/after/IllegalSynonymInstance.hs

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+{-# LANGUAGE Haskell2010 #-}
+module IllegalSynonymInstance where
+
+data RGB = R | G | B
+
+instance Eq RGB where
+    R == R = True
+    G == G = True
+    B == B = True
+    _ == _ = False

message-index/messages/GHC-93557/illegalSynonymInstance/before/IllegalSynonymInstance.hs

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+{-# LANGUAGE Haskell2010 #-}
+module IllegalSynonymInstance where
+
+data RGB = R | G | B
+
+type T = RGB
+
+instance Eq T where
+    R == R = True
+    G == G = True
+    B == B = True
+    _ == _ = False

message-index/messages/GHC-93557/illegalSynonymInstance/index.md

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+---
+title: Cannot implement a typeclass for type synonyms by default
+---
+
+The programmer defined a data type `RGB`, and a type synonym `T`, and then tried to implement an instance of the typeclass `Eq` for `T`.
+In the language version Haskell2010 this is not allowed, since all typeclass instances have to start with a type constructor.
+This problem can be fixed by implementing the typeclass instance directly for the type `RGB` instead of the type synonym `T`.
+
+```
+messages/GHC-93557/illegalSynonymInstance/before/IllegalSynonymInstance.hs:8:10: error: [GHC-93557]
+    • Illegal instance declaration for ‘Eq T’:
+        All instance types must be of the form (T t1 ... tn)
+        where T is not a synonym.
+    • In the instance declaration for ‘Eq T’
+    Suggested fix: Perhaps you intended to use TypeSynonymInstances
+  |
+8 | instance Eq T where
+  |          ^^^^
+```

message-index/messages/GHC-93557/index.md

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+---
+title: Illegal typeclass instance
+summary: Illegal typeclass instance
+severity: error
+introduced: 9.8.1
+---
+
+Using the language version Haskell2010, only certain types are allowed as instances of a typeclass.
+Every typeclass instance in Haskell2010 has the following form:
+```
+instance C t where
+````
+Here, `C` is the name of a typeclass, such as `Eq`, `Show` or `Read`, and `t` is a type.
+But only a subset of types `t` is allowed to appear in typeclass instances.
+These types must all have the form `T` or `T a ... b`, where `T` has to be the name of a type introduced by a data or newtype declaration, and the arguments `a` to `b` all have to be type variables.