Update example for 02256 with better example

Author: BinderDavid

message-index/messages/GHC-02256/accounts/after/Accounting.hs

@@ -0,0 +1,7 @@
+module Accounting where
+
+import qualified AccountingRecords as Personal (PersonalAccount(..))
+import qualified AccountingRecords as Business (BusinessAccount(..))
+
+resetPersonalAccount :: Personal.PersonalAccount -> Personal.PersonalAccount
+resetPersonalAccount account = account { Personal.balance = 0 }

message-index/messages/GHC-02256/accounts/after/AccountingRecords.hs

@@ -0,0 +1,6 @@
+{-# LANGUAGE DuplicateRecordFields #-}
+module AccountingRecords where
+
+data BusinessAccount = MkBusinessAccount { balance :: Int }
+
+data PersonalAccount = MkPersonalAcccount { balance :: Int }

message-index/messages/GHC-02256/accounts/before/Accounting.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE DuplicateRecordFields #-}
+module Accounting where
+
+data BusinessAccount = MkBusinessAccount { balance :: Int }
+
+data PersonalAccount = MkPersonalAccount { balance :: Int }
+
+resetPersonalAccount :: PersonalAccount -> PersonalAccount
+resetPersonalAccount account = account { balance = 0 }

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

@@ -0,0 +1,19 @@
+---
+title: Ambiguous Record Update
+---
+
+In this example we declare two records `BusinessAccount` and `PersonalAccount` which each contain a field `balance` which stores the current balance. In the function `resetPersonalAccount` we use this record field to reset the account balance of a personal account to 0.
+The compiler can successfully detect that the field `balance` that we mention refers to the field of the record `PersonalAccount` and not to the field of the record `BusinessAccount`, but the mechanism that GHC uses for this is brittle and will be deprecated in a future release.
+For this reason GHC emits a warning.
+
+```
+before/Accounting.hs:9:42: warning: [GHC-02256] [-Wambiguous-fields]
+    The record update account
+                        {balance = 0} with type PersonalAccount is ambiguous.
+    This will not be supported by -XDuplicateRecordFields in future releases of GHC.
+  |
+9 | resetPersonalAccount account = account { balance = 0 }
+  |  
+```
+
+In order to make your code compile without warnings you can put the record declarations in a separate module and use qualified imports and names to make it unambiguous which record field you mean.

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

@@ -6,5 +6,5 @@ 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.
+The `DuplicateRecordFields` extension lets you 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.