[Haskell-cafe] A curios monad

Martijn van Steenbergen martijn at van.steenbergen.nl
Fri Dec 12 05:48:37 EST 2008

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.



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