safe array freezing?

[Hal Daume III]

afaik, this is fine.  in fact, i use this idiom very frequently, so if 
it's bad, i've never seen the badness of it.

On Tue, 30 Dec 2003, John Meacham wrote:

> so, A common idiom when using Control.Monad.ST is to do some
> complicated, state using computation to compute a big array which is
> then used purely functionally as a read-only array in the rest of the
> program. 
> 
> to avoid the cost of copying the array at the end, we are forced to use
> 'unsafeFreeze'. it occurs to me that this particular idiom can actually
> be done safely, by adding the following...
> 
> runSTArray :: (forall s . (ST s (STArray s i e))) -> Array i e
> runSTUArray :: (forall s . (ST s (STUArray s i e))) -> UArray i e
> 
> now, since these discard 's' and hence the ability to modify the arrays
> they return, they may safely use the final arrays directly as if they
> were immutable. 
> 
> the routines are also very easy to implement:
> runSTArray st = runST $ st >>= unsafeFreeze
> runSTUArray st = runST $ st >>= unsafeFreeze          
> 
> 
> so, is my logic correct? that runSTArray and runSTUArray need not be
> considered unsafe? if so, perhaps we should add these (or some better
> generalization that someone comes up with) to the standard libraries, as
> it would be nice to be able to use this idiom without resorting to
> trickery. (well, user visible trickery at least)
>         John
> 
> 

-- 
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume