Haskell Platform proposal: Add the vector package

Simon Marlow marlowsd at gmail.com
Mon Jul 16 15:43:55 CEST 2012

On 16/07/2012 13:55, Roman Leshchinskiy wrote:
> Simon Marlow wrote:
>> Just to repeat what I said earlier, I don't see there being any
>> objection to putting unsafeRead with the other unsafe functions in
>> vector, even though technically it is safe.
> Actually, this particular bit probably isn't a problem. I actually
> simplified the example slightly. The real type of unsafeRead is:
> unsafeRead :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> m a
> Hier, m is either IO or ST. What I didn't realise was that runST is marked
> as Trustworthy. This means that for ST-based code to be Trustworthy, it
> must really be safe when executed. This is different from IO where Safe
> Haskell doesn't care what happens when it's executed. I'm not sure if this
> is documented anywhere, it certainly wasn't obvious to me. The end effect
> is that while the IO instantiation of unsafeRead is safe, the ST one
> isn't. Hence, it can't be marked as Trustworthy anyway. All functions on
> mutable vectors are overloaded in this way, so it seems that the presence
> of ST makes Safe Haskell's notion of safety much closer to the one vector
> uses.

Ok, that's good then.

The point about IO is made in the paper, but should probably be more 
clear in the documentation.


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