On 10 November 2004 10:54, Graham Klyne wrote:

At 23:25 09/11/04 +0100, Remi Turk wrote:

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(I looked at Data.HashTable, but I couldn't figure why it needs to be implemented in the IO monad, except to optimize the internal implementation. Also, it's not clear to me how it behaves when a key is inserted that already exists.) A hash-table becomes rather useless without mutable state AFAICS. Without it, one might almost just as well use a list of pairs... Actually, some kind of freezeHashTable may be useful, and a HashTable in the ST monad is definitely useful: I guess patches are welcome..

I can see why using (something like) a state monad might be useful, but not why it needs to be an IO monad, unless there's some fairly low-down optimization being performed.

(I'm not asking for this, BTW, just commenting on the apparent lack. For my application, I am using a list of pairs, as I expect these tables to be relatively small.)

There is no good reason that Data.HashTable needs to be in the IO monad, other than that's where I wanted to use it and having two versions (or using lots of stToIO) would have been a pain. The performance of ST & IO are the same. Long ago, when IO exceptions were implemented explicitly in the IO monad, IO used to be less efficient than ST. Cheers, Simon