[Haskell] STM Monad Transformer
david at overtons.id.au
Thu Jun 8 11:01:23 EDT 2006
Not sure how helpful I can be, but since no-one else has replied...
On 06/06/06, Thomas Conway <drtomc at gmail.com> wrote:
> Consider the following prototypical definitions:
> data Node k v = Leaf [(k,v)] | Node [(k,NodePtr k v)]
> type NodePtr k v = TVar (Either Int (Node k v))
> where the Int is a file offset, or page number, or somesuch.
> If an attempt is made during a STM transaction to access a page that
> is not in memory,
> I'd like the transaction to "abort" and return the address of the page
> so that it can be read from disk and the transaction retried.
> Now, the code is going to contain quite a frew fragments like the following:
> insert p k v
> = do
> n <- getNode p
> case n of
> Leaf ... ->
> I think I want the type of insert to be
> insert :: NodePtr k v -> k -> v -> Either (STM ()) Int
> that is, either I get back the transaction, or the page address.
> The question is, how do I combine the two monads STM and Either to
> achieve this cleanly?
Is there a reason why you can't combine the monads the other way around and
then use ErrorT monad transformer? I.e. something like this (untested code):
type ErrorSTM = ErrorT Int STM
insert :: NodePtr k v -> k -> v -> ErrorSTM ()
getNode :: NodePtr k v -> ErrorSTM (Node k v)
getNode = ErrorT . readTVar
> I've tried reading a few pages about monad transformers, but it is not
> evident to me yet how to combine the two. I think it calls for STMT
> (following the FooT naming convention).
Obviously you can't have a STM transformer because that would allow you to do
IO inside an STM transaction.
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