add list length

src/List.derw

@@ -1,6 +1,8 @@
 import "./Maybe" exposing ( Maybe )
 
-exposing (emptyList, map, filter, foldl, filterMap, append, reverse)
+import "./List_kernel" exposing (kernelLength)
+
+exposing (emptyList, map, filter, foldl, filterMap, append, reverse, length)
 
 emptyList: List any
 emptyList =
@@ -40,4 +42,8 @@ append xs ys =
 
 reverse: List any -> List any
 reverse xs =
-    xs.reverse()
+    xs.reverse()
+
+length: List any -> number
+length xs =
+    kernelLength xs

src/List_kernel.ts

@@ -0,0 +1,3 @@
+export function kernelLength(xs: any[]): number {
+    return xs.length;
+}

src/List_test.derw

@@ -1,7 +1,7 @@
-exposing (testEmptyList, testMap, testFilter, testFoldl, testFilterMap, testAppend, testReverse)
+exposing (testEmptyList, testMap, testFilter, testFoldl, testFilterMap, testAppend, testReverse, testLength)
 
 import "./Test" exposing (equals)
-import "./List" exposing (emptyList, map, filter, foldl, filterMap, append, reverse)
+import "./List" exposing (emptyList, map, filter, foldl, filterMap, append, reverse, length)
 import "./Maybe" exposing (Maybe, Just, Nothing)
 
 testEmptyList: boolean -> void
@@ -96,4 +96,10 @@ testAppend a? =
 testReverse: boolean -> void
 testReverse a? =
     reverse [ 1, 2 ]
-        |> equals [ 2, 1 ]
+        |> equals [ 2, 1 ]
+
+testLength: boolean -> void
+testLength a? =
+    [ 1, 2, 3 ]
+        |> length
+        |> equals 3