gensym in Hy

Simple implementation of gensym in Hy.

Returns a new HySymbol.

Usable in macros like:

(defmacro nif [expr pos zero neg]
  (let [[g (gensym)]]
    `(let [[~g ~expr]]
       (cond [(pos? ~g) ~pos]
             [(zero? ~g) ~zero]
             [(neg? ~g) ~neg]))))

This addresses all the general comments about (gensym), and doesn't
try to implement "auto-gensym" yet. But clearly the macro approach
instead of the pre-processor approach (as described in the
letoverlambda (http://letoverlambda.com/index.cl/guest/chap3.html#sec_5)
is the way to go

hy/core/language.hy

@@ -23,6 +23,7 @@
 ;;;; to make functional programming slightly easier.
 ;;;;
 
+
 (import [hy._compat [long-type]]) ; long for python2, int for python3
 
 (defn _numeric-check [x]
@@ -91,6 +92,20 @@
   "Return True if x is float"
   (isinstance x float))
 
+(import [threading [Lock]])
+(setv _gensym_counter 1234)
+(setv _gensym_lock (Lock))
+
+(defn gensym [&optional [g "G"]]
+  (let [[new_symbol None]]
+    (global _gensym_counter)
+    (global _gensym_lock)
+    (.acquire _gensym_lock)
+    (try (do (setv _gensym_counter (inc _gensym_counter))
+             (setv new_symbol (HySymbol (.format ":{0}_{1}" g _gensym_counter))))
+         (finally (.release _gensym_lock)))
+    new_symbol))
+
 (defn inc [n]
   "Increment n by 1"
   (_numeric-check n)
@@ -223,6 +238,7 @@
   (= n 0))
 
 (def *exports* '[cycle dec distinct drop drop-while empty? even? filter float?
+                 gensym
                  inc instance? integer integer? iterable? iterate iterator? neg?
                  none? nth numeric? odd? pos? remove repeat repeatedly second
                  string string? take take-nth take-while zero?])

hy/core/macros.hy

@@ -113,6 +113,6 @@
 
 (defmacro yield-from [iterable]
   "Yield all the items from iterable"
-  ;; TODO: this needs some gensym love
-  `(foreach [_hy_yield_from_x ~iterable]
-     (yield _hy_yield_from_x)))
+  (let [[x (gensym)]]
+  `(foreach [~x ~iterable]
+     (yield ~x))))

tests/native_tests/core.hy

@@ -141,6 +141,18 @@
   (assert-true (float? -3.2))
   (assert-false (float? "foo")))
 
+(defn test-gensym []
+  "NATIVE: testing the gensym function"
+  (import [hy.models.symbol [HySymbol]])
+  (setv s1 (gensym))
+  (assert (isinstance s1 HySymbol))
+  (assert (= 0 (.find s1 ":G_")))
+  (setv s2 (gensym "xx"))
+  (setv s3 (gensym "xx"))
+  (assert (= 0 (.find s2 ":xx_")))
+  (assert (not (= s2 s3)))
+  (assert (not (= (str s2) (str s3)))))
+
 (defn test-inc []
   "NATIVE: testing the inc function"
   (assert-equal 3 (inc 2))
@@ -393,3 +405,4 @@
   (assert-equal res [None None])
   (setv res (list (take-while (fn [x] (not (none? x))) [1 2 3 4 None 5 6 None 7])))
   (assert-equal res [1 2 3 4]))
+

tests/native_tests/native_macros.hy

@@ -94,7 +94,6 @@
 (assert initialized)
 (assert (test-initialized))
 
-
 (defn test-yield-from []
   "NATIVE: testing yield from"
   (defn yield-from-test []
@@ -107,3 +106,28 @@
   (import sys)
   (assert (= (get sys.version_info 0)
              (if-python2 2 3))))
+
+(defn test-gensym-in-macros []
+  (import ast)
+  (import [astor.codegen [to_source]])
+  (import [hy.importer [import_buffer_to_ast]])
+  (setv macro1 "(defmacro nif [expr pos zero neg]
+      (let [[g (gensym)]]
+        `(let [[~g ~expr]]
+           (cond [(pos? ~g) ~pos]
+                 [(zero? ~g) ~zero]
+                 [(neg? ~g) ~neg]))))
+
+    (print (nif (inc -1) 1 0 -1))
+    ")
+  ;; expand the macro twice, should use a different
+  ;; gensym each time
+  (setv _ast1 (import_buffer_to_ast macro1 "foo"))
+  (setv _ast2 (import_buffer_to_ast macro1 "foo"))
+  (setv s1 (to_source _ast1))
+  (setv s2 (to_source _ast2))
+  ;; and make sure there is something new that starts with :G_
+  (assert (in ":G_" s1))
+  (assert (in ":G_" s2))
+  ;; but make sure the two don't match each other
+  (assert (not (= s1 s2))))