kspeakman · February 14, 2018 21:51 · Feb 14, 2018
diff --git a/ListFolds.elm b/ListFolds.elm
@@ -0,0 +1,43 @@
+module ListFolds exposing (..)
+
+{-| Alternative implementations of List.foldl and List.foldr
+    
+    -- this will add the `fold` and `foldBack` functions to the List module
+    import ListFolds as List
+    
+    List.fold (++) "" [ "Hello", " ", "World", "!" ] == "Hello World!"
+    
+-}
+
+
+{-| Fold a list from the left, properly.
+
+    List.fold (++) "" [ "Hello", " ", "World", "!" ] == "Hello World!"
+
+    -- Compare to built-in `foldl`:
+    List.foldl (++) "" [ "Hello", " ", "World", "!" ] == "!World Hello"
+
+    -- What you actually have to do to use built-in: flip argument order
+    List.foldl (\a b -> b ++ a) "" [ "Hello", " ", "World", "!" ] == "Hello World!"
+
+-}
+fold : (b -> a -> b) -> b -> List a -> b
+fold f b list =
+    List.foldl (flip f) b list
+
+
+{-| Right fold with visual argument order
+
+    -- direction of processing: 4<----------3<----------2<-----------1<
+    List.foldBack (::) [ "Option 3", "Option 2", "Option 1" ] [ "Select an option" ]
+      == [ "Select an option", "Option 1", "Option 2", "Option 3" ]
+
+    --                                  --------------------------------v
+    -- Compare to built-in `foldr`: >1-/       4<----------3<-----------2
+    List.foldr (::) [ "Select an option" ] [ "Option 3", "Option 2", "Option 1" ]
+      == [ "Select an option", "Option 1", "Option 2", "Option 3" ]
+
+-}
+foldBack : (a -> b -> b) -> List a -> b -> b
+foldBack f list b =
+    List.foldr f b list
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