Implement unary minus

hy/compiler.py

@@ -812,7 +812,6 @@ class HyASTCompiler(object):
 
     @builds("+")
     @builds("%")
-    @builds("-")
     @builds("/")
     @builds("//")
     @builds("*")
@@ -851,6 +850,18 @@ class HyASTCompiler(object):
             left = calc
         return calc
 
+    @builds("-")
+    @checkargs(min=1)
+    def compile_maths_expression_sub(self, expression):
+        if len(expression) > 2:
+            return self.compile_maths_expression(expression)
+        else:
+            arg = expression[1]
+            return ast.UnaryOp(op=ast.USub(),
+                               operand=self.compile(arg),
+                               lineno=arg.start_line,
+                               col_offset=arg.start_column)
+
     @builds("+=")
     @builds("/=")
     @builds("//=")

tests/native_tests/math.hy

@@ -17,7 +17,8 @@
 
 (setv test_sub (fn []
   "NATIVE: Test subtraction"
-  (assert (= 4 (- 8 4)))))
+  (assert (= 4 (- 8 4)))
+  (assert (= -8 (- 8)))))
 
 
 (setv test_add (fn []