From d36c55ca07995b3405d5dfc0e96d99a12a2f0cce Mon Sep 17 00:00:00 2001
From: Olivia Brode-Roger <olivia@brilliant.org>
Date: Tue, 11 Jul 2023 16:42:24 -0700
Subject: [PATCH] add reverseFilter

---
 src/List/Extra.elm | 25 +++++++++++++++++++++++--
 tests/Tests.elm    |  5 +++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/List/Extra.elm b/src/List/Extra.elm
index cdf8ba3..3d95253 100644
--- a/src/List/Extra.elm
+++ b/src/List/Extra.elm
@@ -1,5 +1,5 @@
 module List.Extra exposing
-    ( last, init, getAt, uncons, unconsLast, maximumBy, maximumWith, minimumBy, minimumWith, andMap, andThen, reverseMap, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, setIf, setAt, remove, updateIf, updateAt, updateIfIndex, removeAt, removeIfIndex, filterNot, swapAt, stableSortWith
+    ( last, init, getAt, uncons, unconsLast, maximumBy, maximumWith, minimumBy, minimumWith, andMap, andThen, reverseMap, reverseFilter, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, setIf, setAt, remove, updateIf, updateAt, updateIfIndex, removeAt, removeIfIndex, filterNot, swapAt, stableSortWith
     , intercalate, transpose, subsequences, permutations, interweave, cartesianProduct, uniquePairs
     , foldl1, foldr1, indexedFoldl, indexedFoldr, Step(..), stoppableFoldl
     , scanl, scanl1, scanr, scanr1, mapAccuml, mapAccumr, unfoldr, iterate, initialize, cycle, reverseRange
@@ -17,7 +17,7 @@ module List.Extra exposing
 
 # Basics
 
-@docs last, init, getAt, uncons, unconsLast, maximumBy, maximumWith, minimumBy, minimumWith, andMap, andThen, reverseMap, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, setIf, setAt, remove, updateIf, updateAt, updateIfIndex, removeAt, removeIfIndex, filterNot, swapAt, stableSortWith
+@docs last, init, getAt, uncons, unconsLast, maximumBy, maximumWith, minimumBy, minimumWith, andMap, andThen, reverseMap, reverseFilter, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, setIf, setAt, remove, updateIf, updateAt, updateIfIndex, removeAt, removeIfIndex, filterNot, swapAt, stableSortWith
 
 
 # List transformations
@@ -569,6 +569,27 @@ reverseMap f xs =
     foldl (\x acc -> f x :: acc) [] xs
 
 
+{-| `reverseMap f xs` gives the same result as `List.reverse (List.map f xs)`,
+but is tail-recursive and slightly more efficient.
+
+    reverseFilter (\x -> x > 5) [ 1, 4, 9, 16]
+    --> [ 16, 9 ]
+
+-}
+reverseFilter : (a -> Bool) -> List a -> List a
+reverseFilter isGood xs =
+    foldl
+        (\x acc ->
+            if isGood x then
+                x :: acc
+
+            else
+                acc
+        )
+        []
+        xs
+
+
 {-| Negation of `member`.
 
     notMember 1 [ 1, 2, 3 ]
diff --git a/tests/Tests.elm b/tests/Tests.elm
index a362ef7..080a94e 100644
--- a/tests/Tests.elm
+++ b/tests/Tests.elm
@@ -40,6 +40,11 @@ all =
                 \() ->
                     Expect.equal (reverseMap sqrt [ 1, 4, 9 ]) [ 3, 2, 1 ]
             ]
+        , describe "reverseFilter" <|
+            [ test "filters and reverses" <|
+                \() ->
+                    Expect.equal (reverseFilter (\x -> x > 5) [ 1, 4, 9, 16 ]) [ 16, 9 ]
+            ]
         , describe "notMember" <|
             [ test "disconfirms member" <|
                 \() ->