refactor: update purity-reflection.md

src/purity-reflection.md

@@ -13,20 +13,22 @@ For example, here is the implementation of `Set.count`:
 ```flix
 @ParallelWhenPure
 pub def count(f: a -> Bool \ ef, s: Set[a]): Int32 \ ef =
-    typematch f {
-        case g: a -> Bool \ {} =>
-            let h = (k, _) -> g(k);
-            let Set(t) = s;
-            RedBlackTree.parCount(threads() - 1, h, t)
-        case g: a -> Bool \ ef => foldLeft((b, k) -> if (g(k)) b + 1 else b, 0, s)
-        case _: _ => unreachable!()
+    match purityOf(f) {
+        case Purity.Pure(g) =>
+            if (useParallelEvaluation(s))
+                let h = (k, _) -> g(k);
+                let Set(t) = s;
+                RedBlackTree.parCount(h, t)
+            else
+                foldLeft((b, k) -> if (f(k)) b + 1 else b, 0, s)
+        case Purity.Impure(g) => foldLeft((b, k) -> if (g(k)) b + 1 else b, 0, s)
     }
 ```
 
-Here the `typematch` construct is used to reflect on the purity of `f`:
+Here the `purityOf` function is used to reflect on the purity of `f`:
 
 - If `f` is pure then we evaluate `Set.count` in _parallel_ over the elements of
-  the set. 
+  the set (if the set is big enough to warrant it). 
 - If `f` is effectful then we use an ordinary (single-threaded) fold.
 
 The advantage is that we get parallelism for free – _if_ `f` is pure.