Document GHC-02256

Author: BinderDavid

message-index/messages/GHC-02256/ambiguous/after/AmbiguousA.hs

@@ -0,0 +1,7 @@
+{-# LANGUAGE DuplicateRecordFields #-}
+module AmbiguousA where
+
+data Foo = Foo { foo :: Int }
+
+data Bar = Bar { foo :: Int }
+

message-index/messages/GHC-02256/ambiguous/after/AmbiguousB.hs

@@ -0,0 +1,7 @@
+module AmbiguousB where
+
+import qualified AmbiguousA as Foo (Foo(..))
+import qualified AmbiguousA as Bar (Bar(..))
+
+blah :: Bar.Bar -> Bar.Bar
+blah bar = bar { Bar.foo = 5 }

message-index/messages/GHC-02256/ambiguous/before/AmbiguousA.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE DuplicateRecordFields #-}
+module Ambiguous where
+
+data Foo = Foo { foo :: Int }
+
+data Bar = Bar { foo :: Int }
+
+blah :: Bar -> Bar
+blah bar = bar { foo = 5 }

message-index/messages/GHC-02256/ambiguous/index.md

@@ -0,0 +1,17 @@
+---
+title: Ambiguous Record Update
+---
+
+In this example we declare two records `Foo` and `Bar` which each contain a field with the same name `foo`. In the function `blah` we use this name to update the field in a record of type `Bar`.
+The compiler can successfully detect that the field `foo` that we mention in the record update can only refer to the field of the record `Bar`, but the mechanism that GHC uses for this will be deprecated in a future release, and for this reason a warning is emitted.
+
+```
+before/Ambiguous.hs:9:18: warning: [GHC-02256] [-Wambiguous-fields]
+    The record update bar {foo = 5} with type Bar is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+  |
+9 | blah bar = bar { foo = 5 }
+  |     
+```
+
+In order to make your code compile without any warnings you can put the declarations of both records in a separate module, and use qualified imports and qualified names to make it unambiguous which record field you mean.

message-index/messages/GHC-02256/index.md

@@ -0,0 +1,10 @@
+---
+title: Ambiguous record update
+summary: Record update with duplicate field names is ambiguous
+severity: warning
+flag: -Wambiguous-fields
+introduced: 9.6.1
+---
+
+With the `DuplicateRecordFields` extension enabled it is possible to use the same field name in multiple records within the same module.
+This can lead to problems when using field accessors to access a field of a record.