added flow macros

docs/contrib/flow.rst

@@ -0,0 +1,59 @@
+==========
+Flow
+==========
+
+.. versionadded:: 0.10.2
+
+The ``flow`` macros allow a programmer to direct the flow of his program with
+greater ease.
+   
+   
+Macros
+======
+
+.. _guard:
+.. _switch:
+
+guard
+-----
+
+``guard`` allows you to guard against a condition.
+
+
+Usage: `(guard (cond1) (body1) (cond2) (body2) ...)`
+
+Example:
+
+.. code-block:: hy
+
+    (require hy.contrib.flow)
+                
+    (defn army-greeter [age height]
+        (guard
+            (< age 18) (print "You are too young!")
+            (< height 170) (print "You are too small!")
+            True        (print "Welcome aboard!")))
+
+    
+switch
+-----
+
+``switch`` allows you to run code based on the value of a variable.
+A final extra body allows for a default case.
+
+
+Usage: `(switch var (cond1) (body1) (cond2) (body2) ... )`
+
+Example:
+
+.. code-block:: hy
+
+    (require hy.contrib.flow)
+                
+    (defn bmi-commenter [bmi]
+        (switch bmi
+            (<= 18.5) (print "you are underweight!")
+            (<= 25.0) (print "apparently normal")
+            (<= 30) (print "a little too heavy, but ok")
+                     (print  "You are a whale!")))
+    

docs/contrib/index.rst

@@ -10,3 +10,4 @@ Contents:
    anaphoric
    loop
    multi
+   flow

hy/contrib/flow.hy

@@ -0,0 +1,24 @@
+;; Additional flow macros
+
+(defmacro/g! guard [&rest args]
+  (setv g!cond (car args))
+  (setv g!body (car (cdr args)))
+  (setv g!rest (cdr (cdr args)))
+  (if g!rest
+  `(if ~g!cond
+       ~g!body
+       (guard ~@g!rest))
+  `(if ~g!cond
+       ~g!body)))
+
+(defmacro/g! switch [variable &rest args]
+  (setv g!comp (car args))
+  (setv g!body (car (cdr args)))
+  (setv g!rest (cdr (cdr args)))
+  (setv g!cond `(~(car g!comp) ~variable ~@(cdr g!comp)))
+  (if g!rest
+      (if (cdr g!rest)
+        `(if ~g!cond ~g!body (switch ~variable ~@g!rest))
+        `(if  ~g!cond ~g!body ~@g!rest))
+      `(if  ~g!cond ~g!body)))
+