concurrency.md

@@ -20,6 +20,12 @@ brief rationale.
   - `Control.Concurrent.Chan`, `Control.Concurrent.QSem`,
     `Control.Concurrent.QSemN`, `Control.Concurrent.SampleVar`
 
+- We need a thread-safe interface to mutable state, for use in library code that does not otherwise use concurrency.
+  We have two choices:
+
+  - Use `MVar`s.  A non-concurrent implementation might implement them in terms of `IORef`, for example. 
+  - Use STM.  Easier to use, but not entirely trivial to implement, even in a single-threaded implementation, because exceptions have to abort a transaction ([ sample implementation](http://www.haskell.org//pipermail/haskell-prime/2006-March/001108.html)).
+
 - Concurrent foreign calls are required.
   **Rationale**: