Document GHC-61689

Author: tbidne

message-index/messages/GHC-61689/example1/after/A.hs

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+module A (one) where
+
+one :: ()
+one = ()
+
+two :: ()
+two = ()

message-index/messages/GHC-61689/example1/after/SymbolNotExported.hs

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+module SymbolNotExported where
+
+import A (one)

message-index/messages/GHC-61689/example1/before/A.hs

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+module A (one) where
+
+one :: ()
+one = ()
+
+two :: ()
+two = ()

message-index/messages/GHC-61689/example1/before/SymbolNotExported.hs

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+module SymbolNotExported where
+
+import A (one, two)

message-index/messages/GHC-61689/example1/index.md

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+---
+title: Importing an unexported symbol.
+---
+
+## Message
+```
+SymbolNotExported.hs:3:16: error: [GHC-61689]
+    Module ‘A’ does not export ‘two’.
+  |
+3 | import A (one, two)
+  |                ^^^
+```
+
+## Explanation
+
+The module `SymbolNotExported` imports the symbol `two` even though `A` does not export it.
+
+The fix is to remove the bad import.

message-index/messages/GHC-61689/example2/after/A.hs

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+{-# LANGUAGE NoFieldSelectors #-}
+
+module A (Foo (..)) where
+
+data Foo = MkFoo { fint :: Int, fchar :: Char }

message-index/messages/GHC-61689/example2/after/NoFieldSelectors.hs

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+module NoFieldSelectors where
+
+import A (Foo (MkFoo, fint))

message-index/messages/GHC-61689/example2/before/A.hs

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+{-# LANGUAGE NoFieldSelectors #-}
+
+module A (Foo (..)) where
+
+data Foo = MkFoo { fint :: Int, fchar :: Char }

message-index/messages/GHC-61689/example2/before/NoFieldSelectors.hs

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+module NoFieldSelectors where
+
+import A (Foo (MkFoo), fint)

message-index/messages/GHC-61689/example2/index.md

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+---
+title: Importing a field selector that has been disabled with NoFieldSelectors.
+---
+
+## Message
+```
+NoFieldSelectors.hs:3:24: error: [GHC-61689]
+    Module ‘A’ does not export ‘fint’.
+    Suggested fix:
+      Notice that ‘fint’ is a field selector belonging to the type ‘A.Foo’
+      that has been suppressed by NoFieldSelectors.
+  |
+3 | import A (Foo (MkFoo), fint)
+  |                        ^^^^
+```
+
+## Explanation
+
+With `FieldSelectors` (the default), field selectors are exported such that the following two imports are equivalent:
+
+```haskell
+
+import A (Foo (MkFoo, fint))
+import A (Foo (MkFoo), fint)
+```
+
+With `NoFieldSelectors` at the _definition site_ (`A.hs`), the second example is always an error:
+
+```haskell
+import A (Foo (MkFoo), fint)
+```
+
+The fix is to move the selector import to the type or remove it altogether.