Document GHC-54540

Author: David Binder

message-index/messages/GHC-54540/constructorsNotInScope/after/ConstructorsNotInScope.hs

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+{-# LANGUAGE StandaloneDeriving #-}
+module ConstructorsNotInScope where
+
+import System.IO( Handle )
+
+-- It is not possible to derive an instance for the typeclass in this module.

message-index/messages/GHC-54540/constructorsNotInScope/before/ConstructorsNotInScope.hs

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+{-# LANGUAGE StandaloneDeriving #-}
+module ConstructorsNotInScope where
+
+import System.IO( Handle )
+
+deriving instance Eq Handle

message-index/messages/GHC-54540/constructorsNotInScope/index.md

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+---
+title: Cannot derive an instance for an abstract type
+---
+
+The `Handle` type is abstract and does not export its constructors. It is therefore not possible to derive an instance of a typeclass for the `Handle` type in another module. You can only derive an instance of a typeclass in the module where `Handle` is defined.
+
+````
+messages/GHC-54540/constructorsNotInScope/before/ConstructorsNotInScope.hs:6:1: error: [GHC-54540]
+    • Can't make a derived instance of ‘Eq Handle’:
+        The data constructors of ‘Handle’ are not all in scope
+          so you cannot derive an instance for it
+    • In the stand-alone deriving instance for ‘Eq Handle’
+  |
+6 | deriving instance Eq Handle
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+```

message-index/messages/GHC-54540/index.md

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+---
+title: Cannot derive instance without constructors in scope
+summary: It is not possible to derive a typeclass instance if the constructors of the type are not in scope
+severity: error
+introduced: 9.6.1
+---
+
+Deriving an instance of a typeclass for a type is only possible if the constructors of the type are in scope at the point where you want to derive the instance.