Add {-# WARNING #-} to Data.List.{head,tail}

2 files changed, 7 insertions, 0 deletions

diff --git a/libraries/base/GHC/List.hs b/libraries/base/GHC/List.hs
index b8a34e20e1..c5109fcf38 100644
--- a/libraries/base/GHC/List.hs
+++ b/libraries/base/GHC/List.hs
@@ -83,6 +83,8 @@ head (x:_)              =  x
 head []                 =  badHead
 {-# NOINLINE [1] head #-}
 
+{-# WARNING in "x-partial" head "This is a partial function, it throws an error on empty lists. Use pattern matching or Data.List.uncons instead. Consider refactoring to use Data.List.NonEmpty." #-}
+
 badHead :: HasCallStack => a
 badHead = errorEmptyList "head"
 
@@ -129,6 +131,8 @@ tail                    :: HasCallStack => [a] -> [a]
 tail (_:xs)             =  xs
 tail []                 =  errorEmptyList "tail"
 
+{-# WARNING in "x-partial" tail "This is a partial function, it throws an error on empty lists. Replace it with drop 1, or use pattern matching or Data.List.uncons instead. Consider refactoring to use Data.List.NonEmpty." #-}
+
 -- | \(\mathcal{O}(n)\). Extract the last element of a list, which must be
 -- finite and non-empty.
 --
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 9cdbaaa386..76a79abb3f 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -1,6 +1,9 @@
 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
 ## 4.19.0.0 *TBA*
+  * Add `{-# WARNING in "x-partial" #-}` to `Data.List.{head,tail}`.
+    Use `{-# OPTIONS_GHC -Wno-x-partial #-}` to disable it.
+    ([CLC proposal #87](https://github.com/haskell/core-libraries-committee/issues/87) and [#114](https://github.com/haskell/core-libraries-committee/issues/114))
   * Add `Data.List.!?` ([CLC proposal #110](https://github.com/haskell/core-libraries-committee/issues/110))
   * `maximumBy`/`minimumBy` are now marked as `INLINE` improving performance for unpackable
     types significantly.