baislisp.core/count should return 0 on nil input (#760)

Hi,

could you please review fix for `(count nil)` to return 0. Fixes #759.

Rudimentary tests added for `count`. Thanks

Co-authored-by: ikappaki <[email protected]>

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+ * Fix issue with `(count nil)` throwing an exception (#759).
+
 ## [v0.1.0b0]
 ### Added
  * Added rudimentary support for `clojure.stacktrace` with `print-cause-trace` (part of #721)

src/basilisp/core.lpy

@@ -265,8 +265,9 @@
 ;;;;;;;;;;;; full support for syntax quote begins here ;;;;;;;;;;;;
 
 (def
-  ^{:doc      "Return the length of ``coll`` as by Python's ``len`` builtin. If the
-               collection does not respond to ``__len__``\\, then count it manually."
+  ^{:doc      "Return the length of ``coll`` as by Python's ``len`` builtin, or 0 if
+               ``coll`` is nil. If the collection does not respond to ``__len__``\\,
+               then count it manually."
     :arglists '([coll])
     :inline   true}
   count

src/basilisp/lang/runtime.py

@@ -1115,15 +1115,18 @@ def apply_kw(f, args):
 
 
 def count(coll) -> int:
-    try:
-        return len(coll)
-    except (AttributeError, TypeError):
+    if coll is None:
+        return 0
+    else:
         try:
-            return sum(1 for _ in coll)
-        except TypeError as e:
-            raise TypeError(
-                f"count not supported on object of type {type(coll)}"
-            ) from e
+            return len(coll)
+        except (AttributeError, TypeError):
+            try:
+                return sum(1 for _ in coll)
+            except TypeError as e:
+                raise TypeError(
+                    f"count not supported on object of type {type(coll)}"
+                ) from e
 
 
 __nth_sentinel = object()

tests/basilisp/runtime_test.py

@@ -260,6 +260,14 @@ def test_apply():
     )
 
 
+def test_count():
+    assert 0 == runtime.count(None)
+    assert 0 == runtime.count(vec.v())
+    assert 0 == runtime.count("")
+    assert 3 == runtime.count(vec.v(1, 2, 3))
+    assert 3 == runtime.count("123")
+
+
 def test_nth():
     assert None is runtime.nth(None, 1)
     assert "not found" == runtime.nth(None, 4, "not found")