Desugar x <~ uniform([c]) to x <- c This is solution #2 of issue #8.

programs/issue8.zar

@@ -0,0 +1,5 @@
+main:
+  p <~ uniform([1]:[int])        
+  x <~ bernoulli(1/2)
+  y <~ bernoulli(1/2)
+  return x

src/Distributions.hs

@@ -16,7 +16,7 @@ uniform lbl dom =
   where
     n = length dom
     go :: [Tree a] -> Tree a
-    go [] = Hole n -- Shouldn't happen.
+    go [] = error "internal error in Distributions:uniform; please report"
     go [x] = x
     go xs = let m = length xs `div` 2 in
               Split Nothing (go $ take m xs) (go $ drop m xs)

src/TreeInterp.hs

@@ -196,11 +196,15 @@ eval (ECond b e1 e2) st = do
 eval (EPrim x) _ = return $ VPrim x
 
 eval (EUniform e) st = do
-  lbl <- freshLbl
   v <- eval e st
   case v of
     VNil -> error "eval: empty list argument to uniform distribution"
-    _ -> return $ VDist $ uniform lbl $ EVal <$> vlist_list v
+    --note(jgs): Don't generate fresh labels in special case of singleton
+    --lists. This is to address issue #8.
+    VCons v1 VNil -> return $ VDist $ Leaf (EVal v1)
+    _ -> do
+      lbl <- freshLbl
+      return $ VDist $ uniform lbl $ EVal <$> vlist_list v
 
 
 -- | Interp. Commands are interpreted as functions from trees of
@@ -266,7 +270,7 @@ interp (While e c) t =
               b' <- is_true e st'
               return $ if b' then Hole fresh_lbl else Leaf st'
             return $ set_label fresh_lbl t''
-            else
+          else
             return $ Leaf st
     else
       -- Nothing in e depends on randomness so unfold the loop.