[Haskell] STM Monad Transformer

[David Overton]

Hi Tom,

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.

David