Remove car and cdr in favor of first and rest (#1241)

* Remove uses of `car` and `cdr` in /hy

* Remove uses of `car` and `cdr` in quote tests

* Remove `car` and `cdr` in favor of `first` and `rest`

I beefed up the documentation and tests for `first` and `rest` while I was at it.

I defined `car` and `cdr` in native_tests.cons so the tests read a bit more naturally.

docs/language/api.rst

@@ -850,16 +850,26 @@ eval-when-compile
 -----------------
 
 
-first / car
------------
+first
+-----
 
-``first`` and ``car`` are macros for accessing the first element of a collection:
+``first`` is a function for accessing the first element of a collection.
 
 .. code-block:: clj
 
     => (first (range 10))
     0
 
+It is implemented as ``(next (iter coll) None)``, so it works with any
+iterable, and if given an empty iterable, it will return ``None`` instead of
+raising an exception.
+
+.. code-block:: clj
+
+    => (first (repeat 10))
+    10
+    => (first [])
+    None
 
 for
 ---
@@ -1402,17 +1412,23 @@ periods in them. In fact, ``mymodule`` and ``M`` aren't defined by these
 to do introspection of the current module's set of defined macros, which isn't
 really supported anyway.
 
-rest / cdr
-----------
+rest
+----
 
-``rest`` and ``cdr`` return the collection passed as an argument without the
-first element:
+``rest`` takes the given collection and returns an iterable of all but the
+first element.
 
 .. code-block:: clj
 
-    => (rest (range 10))
+    => (list (rest (range 10)))
     [1, 2, 3, 4, 5, 6, 7, 8, 9]
 
+Given an empty collection, it returns an empty iterable.
+
+.. code-block:: clj
+
+    => (list (rest []))
+    []
 
 set-comp
 --------

docs/language/core.rst

@@ -111,10 +111,10 @@ Returns a fresh :ref:`cons cell <hycons>` with car *a* and cdr *b*.
 
    => (setv a (cons 'hd 'tl))
 
-   => (= 'hd (car a))
+   => (= 'hd (get a 0))
    True
 
-   => (= 'tl (cdr a))
+   => (= 'tl (cut a 1))
    True
 
 

hy/contrib/multi.hy

@@ -87,6 +87,9 @@
        (with-decorator (method-decorator ~name)
          (defn ~name ~params ~@body))))
 
+(defn head-tail [l]
+  (, (get l 0) (cut l 1)))
+
 (defmacro defn [name &rest bodies]
   (def arity-overloaded? (fn [bodies]
                            (if (isinstance (first bodies) HyString)
@@ -97,17 +100,14 @@
     (do
      (def comment (HyString))
      (if (= (type (first bodies)) HyString)
-       (do (def comment (car bodies))
-           (def bodies (cdr bodies))))
+       (def [comment bodies] (head-tail bodies)))
      (def ret `(do))
      (.append ret '(import [hy.contrib.multi [MultiDispatch]]))
      (for [body bodies]
-       (def let-binds (car body))
-       (def body (cdr body))
+       (def [let-binds body] (head-tail body))
        (.append ret 
                 `(with-decorator MultiDispatch (defn ~name ~let-binds ~comment ~@body))))
      ret)
     (do
-     (setv lambda-list (first bodies))
-     (setv body (rest bodies))
+     (setv [lambda-list body] (head-tail bodies))
      `(setv ~name (fn ~lambda-list ~@body)))))

hy/core/macros.hy

@@ -59,16 +59,6 @@
     `(do ~@body)))
 
 
-(defmacro car [thing]
-  "Get the first element of a list/cons"
-  `(get ~thing 0))
-
-
-(defmacro cdr [thing]
-  "Get all the elements of a thing, except the first"
-  `(cut ~thing 1))
-
-
 (defmacro cond [&rest branches]
   "shorthand for nested ifs:
    (cond [foo bar] [baz quux]) ->
@@ -87,8 +77,8 @@
          (macro-error branch "cond branches need to be a list"))
        (if (< (len branch) 2)
          (macro-error branch "cond branches need at least two items: a test and one or more code branches"))
-       (setv test (car branch))
-       (setv thebranch (cdr branch))
+       (setv test (first branch))
+       (setv thebranch (cut branch 1))
        `(if ~test (do ~@thebranch)))
 
      (setv root (check-branch branch))

hy/extra/anaphoric.hy

@@ -114,8 +114,8 @@
 (defmacro ap-reduce [form lst &optional [initial-value None]]
   "Anaphoric form of reduce, `acc' and `it' can be used for a form"
   `(do
-    (setv acc ~(if (none? initial-value) `(car ~lst) initial-value))
-    (ap-each ~(if (none? initial-value) `(cdr ~lst) lst)
+    (setv acc ~(if (none? initial-value) `(get ~lst 0) initial-value))
+    (ap-each ~(if (none? initial-value) `(cut ~lst 1) lst)
       (setv acc ~form))
     acc))
 
@@ -142,11 +142,11 @@
                                     (str i)))
                        [i (range 1
                                  ;; find the maximum xi
-                                 (inc (max (+ (list-comp (int (cdr a))
+                                 (inc (max (+ (list-comp (int (cut a 1))
                                                          [a flatbody]
                                                          (and (symbol? a)
                                                               (.startswith a 'x)
-                                                              (.isdigit (cdr a))))
+                                                              (.isdigit (cut a 1))))
                                               [0]))))])
             ;; generate the &rest parameter only if 'xi is present in body
             ~@(if (in 'xi flatbody)

tests/native_tests/cons.hy

@@ -1,3 +1,7 @@
+(defmacro car [x] `(get ~x 0))
+(defmacro cdr [x] `(cut ~x 1))
+
+
 (defn test-cons-mutability []
   "Test the mutability of conses"
   (setv tree (cons (cons 1 2) (cons 2 3)))

tests/native_tests/language.hy

@@ -681,10 +681,11 @@
 
 
 (defn test-first []
-  "NATIVE: test firsty things"
+  "NATIVE: test first"
   (assert (= (first [1 2 3 4 5]) 1))
-  (assert (is (first []) None))
-  (assert (= (car [1 2 3 4 5]) 1)))
+  (assert (= (first (range 10)) 0))
+  (assert (= (first (repeat 10)) 10))
+  (assert (is (first []) None)))
 
 
 (defn test-cut []
@@ -710,7 +711,9 @@
 
 (defn test-rest []
   "NATIVE: test rest"
-  (assert (= (list (rest [1 2 3 4 5])) [2 3 4 5])))
+  (assert (= (list (rest [1 2 3 4 5])) [2 3 4 5]))
+  (assert (= (list (take 3 (rest (iterate inc 8)))) [9 10 11]))
+  (assert (= (list (rest [])) [])))
 
 
 (defn test-importas []

tests/native_tests/quote.hy

@@ -11,8 +11,8 @@
 (defn test-quoted-hoistable []
   "NATIVE: check whether quote works on hoisted things"
   (setv f (quote (if True True True)))
-  (assert (= (car f) (quote if)))
-  (assert (= (cdr f) (quote (True True True)))))
+  (assert (= (get f 0) (quote if)))
+  (assert (= (cut f 1) (quote (True True True)))))
 
 
 (defn test-quoted-macroexpand []
@@ -20,8 +20,8 @@
   (setv q1 (quote (-> a b c)))
   (setv q2 (quasiquote (-> a b c)))
   (assert (= q1 q2))
-  (assert (= (car q1) (quote ->)))
-  (assert (= (cdr q1) (quote (a b c)))))
+  (assert (= (get q1 0) (quote ->)))
+  (assert (= (cut q1 1) (quote (a b c)))))
 
 
 (defn test-quote-dicts []