Lazy.NonEmptyList cons (#143)

* Lazy.NonEmptyList cons * Update changelog Co-authored-by: JordanMartinez <[email protected]> Co-authored-by: Jordan Martinez <[email protected]>
purescript · Sep 27, 2021 · 68bce83 · 68bce83
1 parent 6383c4f
commit 68bce83
Show file tree

Hide file tree

Showing 3 changed files with 9 additions and 0 deletions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -7,6 +7,7 @@ Notable changes to this project are documented in this file. The format is based
 Breaking changes:
 
 New features:
+- Added `cons` for `Lazy.NonEmptyList` (#143 by @matthewleon)
 
 Bugfixes:
 

diff --git a/src/Data/List/Lazy/NonEmpty.purs b/src/Data/List/Lazy/NonEmpty.purs
@@ -11,6 +11,7 @@ module Data.List.Lazy.NonEmpty
   , last
   , tail
   , init
+  , cons
   , uncons
   , length
   , concatMap
@@ -69,6 +70,10 @@ init (NonEmptyList nel) =
     of x :| xs ->
       maybe L.nil (x : _) (L.init xs)
 
+cons :: forall a. a -> NonEmptyList a -> NonEmptyList a
+cons y (NonEmptyList nel) =
+  NonEmptyList (defer \_ -> case force nel of x :| xs -> y :| x : xs)
+
 uncons :: forall a. NonEmptyList a -> { head :: a, tail :: L.List a }
 uncons (NonEmptyList nel) = case force nel of x :| xs -> { head: x, tail: xs }
 

diff --git a/test/Test/Data/List/Lazy.purs b/test/Test/Data/List/Lazy.purs
@@ -441,6 +441,9 @@ testListLazy = do
   log "transpose (singleton nil) == nil"
   assert $ transpose (singleton nil) == (nil :: List (List Int))
 
+  log "NonEmptyList cons should add an element"
+  assert $ NEL.toList (NEL.cons 0 (NEL.singleton 1)) == fromFoldable [0, 1]
+
   log "unfoldr should maintain order"
   assert $ (1..5) == unfoldr step 1