Re: Typesafe MRef with a regular monad

Yes, that's a good point. So there are really three issues: a) single-threaded-ness b) making sure you look up in the right map c) making sure the thing you find has the right type

Even if you have typed keys, (Key a), then if you look them up in the wrong map, any guarantee that it maps to a value of type 'a' is out of the window.

Not true, if you use the finite map implementation based on dynamics. Here, the story is: with single-threaded-ness you can omit the dynamic type checks (they are guaranteed to succeed); if you use finite maps in a persistent manner, this is no longer true. Cheers, Ralf