migration/8.6.md

@@ -24,6 +24,29 @@ Will not typecheck if `StarIsType` is enabled, since `m * n` is treated as if on
 1. Use `*` qualified (e.g., `Proxy (m GHC.TypeLits.* n`). This approach is compliant with the GHC three-release policy, as it does not require CPP to support older GHCs.
 1. Enable the `NoStarIsType` extension. Since `(No)StarIsType` did not exist on older GHCs, this approach will require CPP in order to support older compilers.
 
+### `MonadFailDesugaring` by default
+
+
+GHC now enables the `MonadFailDesugaring` extension by default, as discussed in [ https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail\#Transitionalstrategy](https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Transitionalstrategy). This means that code that uses partial pattern matches in `do`-notation, such as this:
+
+```
+f::[[a]]->[a]f l =do(_:xs)<- l
+  xs
+```
+
+
+Will now desugar to use the `fail` method from the `MonadFail` class instead of from `Monad`. That is, this code will now desugar to something resembling:
+
+```
+importControl.Monad.Failas Fail
+
+f::[[a]]->[a]f l =case l of(_:xs)-> xs
+        _->Fail.fail "Pattern-match failure"
+```
+
+
+Depending on the code, it is possible that this change will result in breakage. See [ https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail\#Adaptingoldcode](https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Adaptingoldcode) for ways to adapt to breakage.
+
 ### Constructor-less GADTs now require `GADTSyntax`