Merge pull request #300 from erdeszt/list-init

Add init function

libraries/List.elm

@@ -7,7 +7,7 @@ list must have the same type.
 @docs (::), (++), isEmpty, length, reverse, map
 
 # Sub-lists
-@docs head, tail, last, filter, take, drop
+@docs head, tail, last, init, filter, take, drop
 
 # Putting Lists Together
 @docs concat, concatMap, join, intersperse, zip, zipWith
@@ -58,6 +58,12 @@ tail = Native.List.tail
 last : [a] -> a
 last = Native.List.last
 
+{-| Extract the elements before the last element in the list. List must be non-empty.
+`(init [1,2,3] == [1,2])
+-}
+init : [a] -> [a]
+init = Native.List.init
+
 {-| Check if a list is empty `(isEmpty [] == True)` -}
 isEmpty : [a] -> Bool
 isEmpty xs =

libraries/Native/List.js

@@ -81,6 +81,19 @@ Elm.Native.List.make = function(elm) {
         return out;
     }
 
+    function init(xs) {
+      if (xs.ctor === '[]') { throwError('init'); }
+      var root = Cons(xs._0, Nil);
+      var curr = root;
+      xs = xs._1;
+      while (xs._1.ctor !== '[]') {
+        curr._1 = Cons(xs._0, Nil)
+        xs = xs._1
+        curr = curr._1
+      }
+      return root;
+    }
+
     function map(f, xs) {
         var arr = [];
         while (xs.ctor !== '[]') {
@@ -278,6 +291,7 @@ Elm.Native.List.make = function(elm) {
         head:head,
         tail:tail,
         last:last,
+        init:init,
 
         map:F2(map),
         foldl:F3(foldl),