Make nth return default value when out of bounds

docs/language/core.rst

@@ -534,9 +534,9 @@ nth
 
 Usage: ``(nth coll n)``
 
-Return the `nth` item in a collection, counting from 0. Unlike
-``get``, ``nth`` works on both iterators and iterables. Raises ``IndexError``
-if the `n` is outside the range of ``coll`` or ``ValueError`` if it's negative.
+Return the `nth` item in a collection, counting from 0. Return ``nil``
+if out of bounds (unless specified otherwise). Raise ``ValueError`` if
+``n`` is negative.
 
 .. code-block:: hy
 
@@ -546,14 +546,21 @@ if the `n` is outside the range of ``coll`` or ``ValueError`` if it's negative.
    => (nth [1 2 4 7] 3)
    7
 
-   => (nth [1 2 4 7] 5)
-   Traceback (most recent call last):
-     ...
-   IndexError: 5
+   => (nil? (nth [1 2 4 7] 5))
+   True
+
+   => (nth [1 2 4 7] 5 "default")
+   'default'
 
    => (nth (take 3 (drop 2 [1 2 3 4 5 6])) 2))
    5
 
+   => (nth [1 2 4 7] -1)
+   Traceback (most recent call last):
+     ...
+   ValueError: Indices for islice() must be None or an integer: 0 <= x <= sys.maxsize.
+
+
 .. _numeric?-fn:
 
 numeric?

hy/core/language.hy

@@ -270,11 +270,10 @@
   (import numbers)
   (instance? numbers.Number x))
 
-(defn nth [coll index]
-  "Return nth item in collection or sequence, counting from 0"
-  (try
-    (next (drop index coll))
-    (catch [e StopIteration] (raise (IndexError index)))))
+(defn nth [coll n &optional [default nil]]
+  "Return nth item in collection or sequence, counting from 0.
+   Return nil if out of bounds unless specified otherwise."
+  (next (drop n coll) default))
 
 (defn odd? [n]
   "Return true if n is an odd number"

tests/native_tests/core.hy

@@ -30,6 +30,9 @@
 (defn assert-equal [x y]
   (assert (= x y)))
 
+(defn assert-nil [x]
+  (assert (is x nil)))
+
 (defn test-coll? []
   "NATIVE: testing coll?"
   (assert-true (coll? [1 2 3]))
@@ -390,13 +393,17 @@
   "NATIVE: testing the nth function"
   (assert-equal 2 (nth [1 2 4 7] 1))
   (assert-equal 7 (nth [1 2 4 7] 3))
-  (try (do (nth [1 2 4 7] 5) (assert False))
-       (catch [e [IndexError]] nil))
+  (assert-nil (nth [1 2 4 7] 5))
+  (assert-equal (nth [1 2 4 7] 5 "some default value")
+                "some default value")  ; with default specified
   (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-nil (nth (iter [1 2 4 7]) 5))
+  (assert-equal (nth (iter [1 2 4 7]) 5 "some default value")
+                "some default value")  ; with default specified
   (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)))

tests/native_tests/language.hy

@@ -495,9 +495,11 @@
   (setv output (list (gen)))
   (assert (= [1] output)))
 
+
 (defn test-first []
   "NATIVE: test firsty things"
   (assert (= (first [1 2 3 4 5]) 1))
+  (assert (is (first []) nil))
   (assert (= (car [1 2 3 4 5]) 1)))