add reverseFilter (#177)

elm-community · Jul 14, 2023 · 882214f · 882214f
1 parent 7542f94
commit 882214f
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diff --git 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
@@ -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" <|
                 \() ->