added xi-forms

These work like Clojure's `#()` anonymous function literals.

docs/contrib/anaphoric.rst

@@ -13,14 +13,11 @@ concise and easy to read.
 
     -- Wikipedia (http://en.wikipedia.org/wiki/Anaphoric_macro)
 
-Macros
-======
-
 
 .. _ap-if:
 
 ap-if
--------
+=====
 
 Usage: ``(ap-if (foo) (print it))``
 
@@ -31,7 +28,7 @@ true and false branches.
 .. _ap-each:
 
 ap-each
--------
+=======
 
 Usage: ``(ap-each [1 2 3 4 5] (print it))``
 
@@ -228,3 +225,24 @@ Returns a function which applies several forms in series from left to right. The
    => (def op (ap-compose (+ it 1) (* it 3)))
    => (op 2)
    9
+
+.. _xi
+
+xi
+==
+
+Usage ``(xi function body ...)``
+
+Returns a function with parameters implicitly determined by the presence in the body of xi parameters. An xi symbol designates the ith parameter (1-based, e.g. x1, x2, x3, etc.), or all remaining parameters for xi itself. This is not a replacement for lambda. The xi forms cannot be nested. 
+
+This is similar to Clojure's anonymous function literals (``#()``).
+
+.. code-block:: hy
+
+   => ((xi identity [x1 x5 [x2 x3] xi x4]) 1 2 3 4 5 6 7 8)
+   [1, 5, [2, 3,] (6, 7, 8), 4]
+   => (def add-10 (xi + 10 x1))
+   => (add-10 6)
+   16
+   
+

hy/contrib/anaphoric.hy

@@ -121,3 +121,27 @@
 (defmacro ap-compose [&rest forms]
   "Returns a function which is the composition of several forms."
   `(fn [var] (ap-pipe var ~@forms)))
+
+(defmacro xi [function &rest body]
+  "Returns a function with parameters implicitly determined by the presence in
+   the body of xi parameters. An xi symbol designates the ith parameter
+   (1-based, e.g. x1, x2, x3, etc.), or all remaining parameters for xi itself.
+   This is not a replacement for lambda. The xi forms cannot be nested. "
+  (setv flatbody (flatten body))
+  `(lambda [;; generate all xi symbols up to the maximum found in body
+            ~@(genexpr (HySymbol (+ "x"
+                                    (str i)))
+                       [i (range 1
+                                 ;; find the maximum xi
+                                 (inc (max (genexpr (int (cdr a))
+                                                    [a flatbody]
+                                                    (and (symbol? a)
+                                                         (.startswith a 'x)
+                                                         (.isdigit (cdr a))))
+                                           :default 0)))])
+            ;; generate the &rest paremeter only if 'xi is present in body
+            ~@(if (in 'xi flatbody)
+                '(&rest xi)
+                '())]
+     (~function ~@body)))
+

tests/native_tests/contrib/anaphoric.hy

@@ -113,3 +113,25 @@
   "NATIVE: testing anaphoric compose"  
   (assert-equal ((ap-compose (+ it 1) (* it 3)) 2) 9)
   (assert-equal ((ap-compose (list (rest it)) (len it)) [4 5 6 7]) 3))
+
+(defn test-xi []
+  "NATIVE: testing xi forms"
+  ;; test ordering
+  (assert-equal ((xi / x1 x2) 2 4) 0.5)
+  (assert-equal ((xi / x2 x1) 2 4) 2)
+  (assert-equal ((xi identity (, x5 x4 x3 x2 x1)) 1 2 3 4 5) (, 5 4 3 2 1))
+  (assert-equal ((xi identity (, x1 x2 x3 x4 x5)) 1 2 3 4 5) (, 1 2 3 4 5))
+  (assert-equal ((xi identity (, x1 x5 x2 x3 x4)) 1 2 3 4 5) (, 1 5 2 3 4))
+  ;; test &rest
+  (assert-equal ((xi sum xi) 1 2 3) 6)
+  (assert-equal ((xi identity (, x1 xi)) 10 1 2 3) (, 10 (, 1 2 3)))
+  ;; no parameters
+  (assert-equal ((xi list)) [])
+  (assert-equal ((xi identity "Hy!")) "Hy!")
+  (assert-equal ((xi identity "xi")) "xi")
+  (assert-equal ((xi + "Hy " "world!")) "Hy world!")
+  ;; test skipped parameters
+  (assert-equal ((xi identity [x3 x1]) 1 2 3) [3 1])
+  ;; test nesting
+  (assert-equal ((xi identity [x1 (, x2 [x3] "Hy" [xi])]) 1 2 3 4 5)
+                [1 (, 2 [3] "Hy" [(, 4 5)])]))