[simonmar]

The memo table (map) is a bit of state that is maintained throughout the request's lifetime. When we compute a memoized value, it is inserted into the map, and if we need the value again we can just grab it from the map instead of recomputing it.

The "automatic" bit is that we insert the code that consults the map so the programmer doesn't have to write it. The map itself is already invisible, because it's inside the monad. So the overall effect is a form of automatic memoization.