Reimplement some built-ins in terms of the standard library.

As a result:

  * functions such as `nth` should work correctly on iterators;
  * `nth` will raise `IndexError` (in a fashion consistent with `get`)
    when the index is out of bounds;
  * `take`, etc. will raise `ValueError` instead of returning
    an ambiguous value if the index is negative;
  * `map`, `zip`, `range`, `input`, `filter` work the same way (Py3k one)
    on both Python 2 and 3 (see #523 and #331).

docs/language/core.rst

@@ -495,8 +495,8 @@ nth
 Usage: ``(nth coll n)``
 
 Return the `nth` item in a collection, counting from 0. Unlike
-``get``, ``nth`` works on both iterators and iterables. Returns ``None``
+``get``, ``nth`` works on both iterators and iterables. Raises ``IndexError``
-if the `n` is outside the range of `coll`.
+if the `n` is outside the range of ``coll`` or ``ValueError`` if it's negative.
 
 .. code-block:: hy
 
@@ -506,8 +506,10 @@ if the `n` is outside the range of `coll`.
    => (nth [1 2 4 7] 3)
    7
 
-   => (none? (nth [1 2 4 7] 5))
+   => (nth [1 2 4 7] 5)
-   True
+   Traceback (most recent call last):
+     ...
+   IndexError: 5
 
    => (nth (take 3 (drop 2 [1 2 3 4 5 6])) 2))
    5
@@ -764,6 +766,7 @@ drop
 Usage: ``(drop n coll)``
 
 Return an iterator, skipping the first ``n`` members of ``coll``
+Raises ``ValueError`` if ``n`` is negative.
 
 .. code-block:: hy
 
@@ -924,6 +927,7 @@ take
 Usage: ``(take n coll)``
 
 Return an iterator containing the first ``n`` members of ``coll``.
+Raises ``ValueError`` if ``n`` is negative.
 
 .. code-block:: hy
 

hy/core/language.hy

@@ -23,7 +23,9 @@
 ;;;; to make functional programming slightly easier.
 ;;;;
 
-
+(import itertools)
+(import functools)
+(import collections)
 (import [hy._compat [long-type]]) ; long for python2, int for python3
 (import [hy.models.cons [HyCons]])
 
@@ -49,15 +51,6 @@
   (and (instance? (type :foo) k)
        (.startswith k (get :foo 0))))
 
-(defn cycle [coll]
-  "Yield an infinite repetition of the items in coll"
-  (setv seen [])
-  (for* [x coll]
-    (yield x)
-    (.append seen x))
-  (while seen
-    (for* [x seen]
-      (yield x))))
 
 (defn dec [n]
   "Decrement n by 1"
@@ -87,22 +80,36 @@
          (yield val)
          (.add seen val))))))
 
+(if-python2
+  (do
+    (setv filterfalse itertools.ifilterfalse)
+    (setv zip_longest itertools.izip_longest)
+    (setv filter itertools.ifilter)
+    (setv map itertools.imap)
+    (setv zip itertools.izip)
+    (setv range xrange)
+    (setv input raw_input))
+  (do
+    (setv reduce functools.reduce)
+    (setv filterfalse itertools.filterfalse)
+    (setv zip_longest itertools.zip_longest)
+    ; Someone can import these directly from `hy.core.language`;
+    ; we'll make some duplicates.
+    (setv filter filter)
+    (setv map map)
+    (setv zip zip)
+    (setv range range)
+    (setv input input)))
+
+(setv cycle itertools.cycle)
+(setv repeat itertools.repeat)
+(setv drop-while itertools.dropwhile)
+(setv take-while itertools.takewhile)
+(setv zipwith map)
+
 (defn drop [count coll]
   "Drop `count` elements from `coll` and yield back the rest"
-  (let [[citer (iter coll)]]
+  (itertools.islice coll count nil))
-    (try (for* [i (range count)]
-           (next citer))
-         (catch [StopIteration]))
-    citer))
-
-(defn drop-while [pred coll]
-  "Drop all elements of `coll` until `pred` is False"
-  (let [[citer (iter coll)]]
-    (for* [val citer]
-      (if (not (pred val))
-        (do (yield val) (break))))
-    (for* [val citer]
-      (yield val))))
 
 (defn empty? [coll]
   "Return True if `coll` is empty"
@@ -126,13 +133,6 @@
         (if (not (hasattr tree attr))
           (setattr tree attr 1))))
 
-(defn filter [pred coll]
-  "Return all elements from `coll` that pass `pred`"
-  (let [[citer (iter coll)]]
-    (for* [val citer]
-      (if (pred val)
-        (yield val)))))
-
 (defn flatten [coll]
   "Return a single flat list expanding all members of coll"
   (if (coll? coll)
@@ -174,7 +174,7 @@
 
 (defn first [coll]
   "Return first item from `coll`"
-  (get coll 0))
+  (nth coll 0))
 
 (defn identity [x]
   "Returns the argument unchanged"
@@ -199,14 +199,13 @@
 (defn integer-char? [x]
   "Return True if char `x` parses as an integer"
   (try
-   (integer? (int x))
+    (integer? (int x))
-   (catch [e ValueError] False)
+    (catch [e ValueError] False)
-   (catch [e TypeError] False)))
+    (catch [e TypeError] False)))
 
 (defn iterable? [x]
   "Return true if x is iterable"
-  (try (do (iter x) true)
+  (isinstance x collections.Iterable))
-       (catch [Exception] false)))
 
 (defn iterate [f x]
   (setv val x)
@@ -216,8 +215,7 @@
 
 (defn iterator? [x]
   "Return true if x is an iterator"
-  (try (= x (iter x))
+  (isinstance x collections.Iterator))
-       (catch [TypeError] false)))
 
 (defn list* [hd &rest tl]
   "Return a dotted list construed from the elements of the argument"
@@ -258,13 +256,9 @@
 
 (defn nth [coll index]
   "Return nth item in collection or sequence, counting from 0"
-  (if (not (neg? index))
+  (try
-    (if (iterable? coll)
+    (next (drop index coll))
-      (try (get (list (take 1 (drop index coll))) 0)
+    (catch [e StopIteration] (raise (IndexError index)))))
-           (catch [IndexError] None))
-      (try (get coll index)
-           (catch [IndexError] None)))
-    None))
 
 (defn odd? [n]
   "Return true if n is an odd number"
@@ -285,14 +279,7 @@
 
 (defn rest [coll]
   "Get all the elements of a coll, except the first."
-  (slice coll 1))
+  (drop 1 coll))
-
-(defn repeat [x &optional n]
-  "Yield x forever or optionally n times"
-  (if (none? n)
-    (setv dispatch (fn [] (while true (yield x))))
-    (setv dispatch (fn [] (for* [_ (range n)] (yield x)))))
-  (dispatch))
 
 (defn repeatedly [func]
   "Yield result of running func repeatedly"
@@ -301,7 +288,7 @@
 
 (defn second [coll]
   "Return second item from `coll`"
-  (get coll 1))
+  (nth coll 1))
 
 (defn some [pred coll]
   "Return true if (pred x) is logical true for any x in coll, else false"
@@ -322,9 +309,7 @@
 (defn take [count coll]
   "Take `count` elements from `coll`, or the whole set if the total
     number of entries in `coll` is less than `count`."
-  (let [[citer (iter coll)]]
+  (itertools.islice coll nil count))
-    (for* [_ (range count)]
-      (yield (next citer)))))
 
 (defn take-nth [n coll]
   "Return every nth member of coll
@@ -337,29 +322,15 @@
           (next citer))))
     (raise (ValueError "n must be positive"))))
 
-(defn take-while [pred coll]
-  "Take all elements while `pred` is true"
-  (let [[citer (iter coll)]]
-    (for* [val citer]
-      (if (pred val)
-        (yield val)
-        (break)))))
-
 (defn zero? [n]
   "Return true if n is 0"
   (_numeric_check n)
   (= n 0))
 
-(defn zipwith [func &rest lists]
-  "Zip the contents of several lists and map a function to the result"
-  (do
-    (import functools)
-    (map (functools.partial (fn [f args] (apply f args)) func) (apply zip lists))))
-
 (def *exports* '[calling-module-name coll? cons cons? cycle dec distinct
                  disassemble drop drop-while empty? even? every? first filter
                  flatten float? gensym identity inc instance? integer
                  integer? integer-char? iterable? iterate iterator? keyword?
-                 list* macroexpand macroexpand-1 neg? nil? none? nth
+                 list* macroexpand macroexpand-1 map neg? nil? none? nth
-                 numeric? odd? pos? remove repeat repeatedly rest second
+                 numeric? odd? pos? range remove repeat repeatedly rest second
-                 some string string? take take-nth take-while zero? zipwith])
+                 some string string? take take-nth take-while zero? zip zipwith])

tests/native_tests/core.hy

@@ -82,8 +82,8 @@
   (assert-equal res [None 4 5])
   (setv res (list (drop 0 [1 2 3 4 5])))
   (assert-equal res [1 2 3 4 5])
-  (setv res (list (drop -1 [1 2 3 4 5])))
+  (try (do (list (drop -1 [1 2 3 4 5])) (assert False))
-  (assert-equal res [1 2 3 4 5])
+       (catch [e [ValueError]] nil))
   (setv res (list (drop 6 (iter [1 2 3 4 5]))))
   (assert-equal res [])
   (setv res (list (take 5 (drop 2 (iterate inc 0)))))
@@ -335,12 +335,15 @@
   "NATIVE: testing the nth function"
   (assert-equal 2 (nth [1 2 4 7] 1))
   (assert-equal 7 (nth [1 2 4 7] 3))
-  (assert-true (none? (nth [1 2 4 7] 5)))
+  (try (do (nth [1 2 4 7] 5) (assert False))
-  (assert-true (none? (nth [1 2 4 7] -1)))
+       (catch [e [IndexError]] nil))
+  (try (do (nth [1 2 4 7] -1) (assert False))
+       (catch [e [ValueError]] nil))
   ;; now for iterators
   (assert-equal 2 (nth (iter [1 2 4 7]) 1))
   (assert-equal 7 (nth (iter [1 2 4 7]) 3))
-  (assert-true  (none? (nth (iter [1 2 4 7]) -1)))
+  (try (do (nth (iter [1 2 4 7]) -1) (assert False))
+       (catch [e [ValueError]] nil))
   (assert-equal 5 (nth (take 3 (drop 2 [1 2 3 4 5 6])) 2)))
 
 (defn test-numeric? []
@@ -429,8 +432,8 @@
   (assert-equal res ["s" "s" "s" "s"])
   (setv res (list (take 0 (repeat "s"))))
   (assert-equal res [])
-  (setv res (list (take -1 (repeat "s"))))
+  (try (do (list (take -1 (repeat "s"))) (assert False))
-  (assert-equal res [])
+       (catch [e [ValueError]] nil))
   (setv res (list (take 6 [1 2 None 4])))
   (assert-equal res [1 2 None 4]))
 

tests/native_tests/language.hy

@@ -474,7 +474,7 @@
 
 (defn test-rest []
   "NATIVE: test rest"
-  (assert (= (rest [1 2 3 4 5]) [2 3 4 5])))
+  (assert (= (list (rest [1 2 3 4 5])) [2 3 4 5])))
 
 
 (defn test-importas []