Add singleton to NonEmpty in libraries/base

This adds a definition to construct a singleton non-empty list
(Data.List.NonEmpty) according to issue #17851.

libraries/base/Data/List/NonEmpty.hs

@@ -42,6 +42,7 @@ module Data.List.NonEmpty (
    , tail        -- :: NonEmpty a -> [a]
    , last        -- :: NonEmpty a -> a
    , init        -- :: NonEmpty a -> [a]
+   , singleton   -- :: a -> NonEmpty a
    , (<|), cons  -- :: a -> NonEmpty a -> NonEmpty a
    , uncons      -- :: NonEmpty a -> (a, Maybe (NonEmpty a))
    , unfoldr     -- :: (a -> (b, Maybe a)) -> a -> NonEmpty b
@@ -168,6 +169,12 @@ last ~(a :| as) = List.last (a : as)
 init :: NonEmpty a -> [a]
 init ~(a :| as) = List.init (a : as)
 
+-- | Construct a 'NonEmpty' list from a single element.
+--
+-- @since 4.15
+singleton :: a -> NonEmpty a
+singleton a = a :| []
+
 -- | Prepend an element to the stream.
 (<|) :: a -> NonEmpty a -> NonEmpty a
 a <| ~(b :| bs) = a :| b : bs

libraries/base/changelog.md

@@ -9,6 +9,9 @@
   * Add `hGetContents'`, `getContents'`, and `readFile'` in `System.IO`:
     Strict IO variants of `hGetContents`, `getContents`, and `readFile`.
 
+  * Add `singleton` function for `Data.List.NonEmpty`.
+
+
 ## 4.14.0.0 *TBA*
   * Bundled with GHC 8.10.1