Reimplement butlast in terms of drop-last

docs/language/core.rst

@@ -6,6 +6,31 @@ Hy Core
 Core Functions
 ===============
 
+.. _butlast-fn:
+
+butlast
+-------
+
+Usage: ``(butlast coll)``
+
+Returns an iterator of all but the last item in ``coll``.
+
+.. code-block:: hy
+
+   => (list (butlast (range 10)))
+   [0, 1, 2, 3, 4, 5, 6, 7, 8]
+
+   => (list (butlast [1]))
+   []
+
+   => (list (butlast []))
+   []
+
+   => (import itertools)
+   => (list (take 5 (butlast (itertools.count 10))))
+   [10, 11, 12, 13, 14]
+
+
 .. _is-coll-fn:
 
 coll?

hy/core/language.hy

@@ -38,7 +38,7 @@
 
 (defn butlast [coll]
   "Returns coll except of last element."
-  (itertools.islice coll 0 (dec (len coll))))
+  (drop-last 1 coll))
 
 (defn coll? [coll]
   "Checks whether item is a collection"

tests/native_tests/core.hy

@@ -38,6 +38,19 @@
   (assert-false (coll? "abc"))
   (assert-false (coll? 1)))
 
+(defn test-butlast []
+  "NATIVE: testing butlast function"
+  (assert-equal (list (butlast (range 10)))
+                [0 1 2 3 4 5 6 7 8])
+  (assert-equal (list (butlast [1]))
+                [])
+  (assert-equal (list (butlast []))
+                [])
+  ; with an infinite sequence
+  (import itertools)
+  (assert-equal (list (take 5 (butlast (itertools.count 10))))
+                [10 11 12 13 14]))
+
 (defn test-cycle []
   "NATIVE: testing cycle"
   (assert-equal (list (cycle [])) [])