I just had a random idea, which I thought I'd share with you all. I've heard about systems that do "copy on write". That is, all users share a single copy of some structure, until somebody tries to write on it. At that moment they get a personal copy to modify so they don't upset everybody else. This way, you don't have to go to all the expense of copying a potentially large structure unless it's actually necessary to do so. Then I started thinking about Clean's "uniqueness typing". If I'm understanding this correctly, it lets you do things like mutate data in-place without requiring a monad. The restriction is that the compiler has to be satisfied, at compile-time, that you will never try to access the old data you just overwrote. Although I once held more optimistic beliefs, as far as I can tell no current, real-world Haskell implementation ever mutates the fields of algebraic data types in-place. (Ignoring for a moment the issue of overwriting a thunk with it's result.) This strikes me as rather pessimistic. I was thinking... If Haskell had uniqueness typing, maybe the compiler could be made to infer when values are used in a unique way. And maybe if the compiler can detect data being used in a unique way, it could code for in-place updates instead of copying, and thereby gain a performance advantage. I don't know how viable this would be - the thing that immediately jumps out at me is that in practice there might be comparatively few places where you can *guarantee* the transformation is safe, and so it might not get applied very much. And considering all this would likely take a fair bit of work on the compiler, that wouldn't be a great payoff. Still, the idea of the compiler transparently rewriting your pure functional code to efficient mutable updates (when safe) is very appealing for performance reasons. Thoughts, people? [I'd be surprised if I'm the first person ever to have this idea...]