Andrew Coppin wrote:
In other words, you can store a value (of arbitrary type) under a unique
key. The monad chooses what key for you, and tells you the key so you
can look up or alter the value again later. Under the covers, it uses
Data.Map to store stuff. I used some trickery with existential
quantification and unsafeCoerce (!!) to make it work. Notice the sneaky
phantom type in the key, telling the type system what type to coerce the
value back to when you read it. Neat, eh?
I did exactly that in my Yogurt project[1]. It felt dirty but in a good
way, mostly because the interface was exactly what I needed. :-)
...until I realised that somebody that somebody could generate a key in
one run and then try to use it in another run. o_O
I've worried about this but I couldn't find a good code example of when
this goes wrong. Can you? Without using any of the unsafeXxx functions,
of course.
Maybe I should build my monad on top of the ST monad if that makes
things safer.
Martijn.
[1]
http://hackage.haskell.org/packages/archive/Yogurt/0.2/doc/html/Network-Yogurt-Mud.html#5
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