+
+\begin{code}
+-- | The Session is a handle to the complete state of a compilation
+-- session. A compilation session consists of a set of modules
+-- constituting the current program or library, the context for
+-- interactive evaluation, and various caches.
+newtype Session = Session (IORef HscEnv)
+\end{code}
+
+HscEnv is like Session, except that some of the fields are immutable.
+An HscEnv is used to compile a single module from plain Haskell source
+code (after preprocessing) to either C, assembly or C--. Things like
+the module graph don't change during a single compilation.
+
+Historical note: "hsc" used to be the name of the compiler binary,
+when there was a separate driver and compiler. To compile a single
+module, the driver would invoke hsc on the source code... so nowadays
+we think of hsc as the layer of the compiler that deals with compiling
+a single module.