Gabriel Scherer wrote
> it is safe to Obj.magic a mutable data-structure into an immutable
> one. The Obj.magic-using code for List.map, implemented in Extlib and
> inherited by Batteries, is careful to use an unsafe cast in exactly
> the second situation. This is a feature that other languages
> (eg. Mezzo) safely provide.

Let me relay a short anecdote about the List.map function in
Batteries. Once I received a message from a person who was using the
Hansei library of probabilistic programming. He reported a problem:
his program produced clearly wrong results. He even traced the problem
to the function 'List.map' -- saying that if he wrote List.map himself,
the problem disappeared. I was initially puzzled: how one can possibly
mis-write List.map. I asked for more details and he said that he was
using Batteries. That gave me a hunch: I went to the Battery
repository to look at the source code and found what I expected --
mutation. In probabilistic programs, mutation has to be done
carefully. Mutation to the global heap forces correlation on what
should be independent ``possible worlds''. Mutation has to be done
with world-local heaps, which Hansei provides for that purpose. There
are times when hidden optimizations come to bite us.

I guess in Mezzo I would have seen from the type that a mutation is
performed, and would have avoided the optimized Map (or the type
checker would make me avoid it).