Added aslice macro (#1249)

Hi,

can you please consider patch to introduce the `aslice` macro in
`basilisp.core`. It addresses #1248.

Thanks

---------

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

Author: ikappaki

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  * Added support for proxies (#425)
  * Added a `:slots` meta flag for `deftype` to disable creation of `__slots__` on created types (#1241)
  * Added support for f-strings (#922)
+ * Added the `aslice` macro to facilitate the use of Python style `array[start:stop:step]` slicing in Basilisp (#1248)
 
 ### Changed
  * Removed implicit support for single-use iterables in sequences, and introduced `iterator-seq` to expliciltly handle them (#1192)

docs/pyinterop.rst

@@ -288,15 +288,30 @@ Type hints may be applied to :lpy:form:`def` names, function arguments and retur
 Python Slicing
 --------------
 
-Python slicing lets you extract parts of a sequence (like a list or string) using the syntax ``sequence[start:end:step]``:
+Python slicing lets you extract parts of a sequence (like a list or string) using the syntax ``sequence[start:stop:step]``:
 
 .. code-block:: python
 
    coll = [-3, -2, -1, 0, 1, 2, 3]
    coll[1:5:2]
    # => [-2, 0]
 
-Basilisp does not support slicing syntax directly, but it can be achieved using the :external:py:obj:`slice` operator combined with the :lpy:fn:`basilisp.core/aget` function:
+Basilisp provides the :lpy:fn:`basilisp.core/aslice` macro to facilitate this syntax:
+
+.. code-block:: clojure
+
+   (def coll #py [-3 -2 -1 0 1 2 3])
+   (aslice coll 3)
+   ;; => #py [-3 -2 -1]
+
+   (aslice coll nil -3)
+   ;; => #py [-3 -2 -1 0]
+
+   (aslice coll 1 5 2)
+   ;; => #py [-2 0]
+
+
+This macro is just a wrapper around Python's :external:py:obj:`slice` operator combined with the :lpy:fn:`basilisp.core/aget` function:
 
 .. code-block:: clojure
 

src/basilisp/core.lpy

@@ -1949,7 +1949,7 @@
   (python/len array))
 
 (defmacro amap
-  "Map ``expr`` over the Python list `array`, returning a new Python list with the
+  "Map ``expr`` over the Python list ``array``, returning a new Python list with the
   result.
 
   This macro initially binds the symbol named by ``ret`` to a clone of ``array``\\. On
@@ -1977,6 +1977,17 @@
          (recur ~expr (inc ~idx))
          ~ret))))
 
+(defmacro aslice
+  "Get a portion of the Python list ``array`` using a
+  Python :external:py:obj:`slice` instance with a single ``stop``
+  value, or a combination of ``start`` and ``stop`` and optional
+  ``step`` values.
+
+  Equivalent to Python's ``array[start:stop:step]`` extended indexing
+  syntax. Use ``nil`` when omitting any of these fields."
+  [array & stop-or-start-stop-step]
+  `(aget ~array (python/slice ~@stop-or-start-stop-step) ))
+
 (defn ^:inline booleans
   "Dummy cast to a Python list of booleans.
 

tests/basilisp/test_core_macros.lpy

@@ -385,6 +385,19 @@
   (let [l #py [1 2 3]]
     (is (= 6 (areduce l idx ret 0 (+ ret (aget l idx)))))))
 
+(deftest aslice-test
+  (let [l #py [-3 -2 -1 0 1 2 3]]
+    (is (= #py [-3 -2 -1 0] (aslice l 4)))
+    (is (= #py [-3 -2 -1 0 1 2 3] (aslice l nil)))
+    (is (= #py [-3 -2 -1 0 1] (aslice l -2)))
+    (is (= #py [-1 0] (aslice l 2 4)))
+    (is (= #py [-3 -2 -1 0] (aslice l nil 4)))
+    (is (= #py [1 2 3] (aslice l 4 nil)))
+    (is (= #py [1 2] (aslice l 4 -1)))
+    (is (= #py [-2 0 2] (aslice l 1 6 2)))
+    (is (= #py [3 1 -1] (aslice l 6 1 -2)))
+    (is (= #py [3 1 -1 -3] (aslice l nil nil -2)))))
+
 (defmacro case-with-seq
   []
   `(case :val