[Haskell-cafe] FiniteMap-like module for unordered keys?

Simon Marlow simonmar at microsoft.com
Thu Nov 11 07:21:30 EST 2004

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

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


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