Document GHC 91999 and GHC 86639

message-index/messages/GHC-86639/example1/after/AddAFunctor.hs

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+{-# LANGUAGE DeriveFunctor #-}
+module AddAFunctor where
+
+data MyFunctor a = MyFunctor a a
+  deriving Functor

message-index/messages/GHC-86639/example1/before/AddAFunctor.hs

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+module AddAFunctor where
+
+data MyFunctor a = MyFunctor a a
+  deriving Functor

message-index/messages/GHC-86639/example1/index.md

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+---
+title: Require the DerivingFunctor language extension for Functor instances
+---
+
+When deriving a `Functor` instance for a datatype in Haskell2010, the
+`DerivingFunctor` language extension must be turned on. This can be done with a
+pragma at the top of the file, or in build settings.
+
+The `DerivingFunctor` extension is turned on automatically from Haskell2021
+onwards.

message-index/messages/GHC-86639/index.md

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+---
+title: Cannot derive an instance without a corresponding language extension
+summary: A language extension must be turned on for a typeclass to be derived.
+severity: error
+introduced: 9.6.1
+---
+
+Some typeclasses can be derived automatically in the `deriving` declaration, but
+require a certain language extension to be turned on.
+
+For example, to derive `Functor` on a datatype, the `DeriveFunctor` language
+extension must be turned on.
+
+There are many available language extensions for deriving instances, the
+appropriate extension depends on the typeclass -- for example, deriving `Functor`
+requires the `DeriveFunctor` language extension, deriving `Generic` requires
+`DeriveGeneric`, and deriving `Lift` requires `DeriveLift`, among others.
+
+There is more info on deriving instances for extra typeclasses in the [GHC user
+manual](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/deriving_extra.html).

message-index/messages/GHC-91999/example1/after/BootModuleDataTypeNotInOrdinaryModule.hs

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+module BootModuleDataTypeNotInOrdinaryModule where
+
+data BootModuleOnlyDataType = BootModuleOnlyDataType String

message-index/messages/GHC-91999/example1/after/BootModuleDataTypeNotInOrdinaryModule.hs-boot

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+module BootModuleDataTypeNotInOrdinaryModule where
+
+data BootModuleOnlyDataType = BootModuleOnlyDataType String

message-index/messages/GHC-91999/example1/after/ImportBootModule.hs

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+module ImportBootModule where
+
+import {-# SOURCE #-} BootModuleDataTypeNotInOrdinaryModule

message-index/messages/GHC-91999/example1/before/BootModuleDataTypeNotInOrdinaryModule.hs

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+module BootModuleDataTypeNotInOrdinaryModule where
+
+-- Defined only in the .hs-boot version of this module
+-- data BootModuleOnlyDataType = BootModuleOnlyDataType String

message-index/messages/GHC-91999/example1/before/BootModuleDataTypeNotInOrdinaryModule.hs-boot

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+module BootModuleDataTypeNotInOrdinaryModule where
+
+data BootModuleOnlyDataType = BootModuleOnlyDataType String

message-index/messages/GHC-91999/example1/before/ImportBootModule.hs

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+module ImportBootModule where
+
+import {-# SOURCE #-} BootModuleDataTypeNotInOrdinaryModule

message-index/messages/GHC-91999/example1/index.md

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+---
+title: Boot module has a datatype which isn't in its corresponding ordinary module
+---
+
+In this example, we define a datatype `BootModuleOnlyDataType` in
+`BootModuleDataTypeNotInOrdinaryModule.hs-boot`.
+
+Compile the example files like so:
+
+```sh
+ghc -c BootModuleDataTypeNotInOrdinaryModule.hs-boot
+ghc -c ImportBootModule.hs
+ghc -c BootModuleDataTypeNotInOrdinaryModule.hs
+```
+
+In the before case, where we defined the datatype `BootModuleOnlyDataType` only
+in `BootModuleDataTypeNotInOrdinaryModule.hs-boot`, we compile the modules like
+so and receive the following error:
+
+```
+<no location info>: error:
+    ‘BootModuleDataTypeNotInOrdinaryModule.BootModuleOnlyDataType’ is exported by the hs-boot file, but not exported by the module
+```
+
+In the after case, we add the datatype to
+`BootModuleDataTypeNotInOrdinaryModule.hs`, and the error disappears.

message-index/messages/GHC-91999/index.md

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+---
+title: Missing Boot Export
+summary: An hs-boot file exports a datatype that isn't found in its corresponding .hs file
+severity: error
+introduced: 9.8.1
+---
+
+Any datatype that is exported from an .hs-boot file must be exported from the
+corresponding .hs file.
+
+More information on .hs-boot modules is available in [the GHC user manual](https://downloads.haskell.org/ghc/latest/docs/users_guide/separate_compilation.html#how-to-compile-mutually-recursive-modules).