[Haskell-cafe] question about STM and IO

[John Lato]

I take it that this follows from the lack of any mechanism to rollback
IO?  If so, I think that the following guidelines suffice for when
it's acceptable to use unsafeIOtoSTM:

1.  The IO action must be able to be safely repeated.
2.  The IO action must be able to be safely performed with
possibly-incorrect arguments, even if it isn't supposed to be
performed.
3.  Don't try to nest transactions.

If I understand it correctly, I think that covers it.
Thanks to everyone who answered; I really appreciate it.
John

On Wed, Feb 20, 2008 at 8:02 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> On 2/20/08, Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:
>  > STM operations can be repeated if first transaction was unsuccessful.
>  > so, you may se here only operations that may be safely repeated - say,
>  > reading/writing memory areas, or reading/writing files, or even
>  > sending network message as long as its duplication is ok
>
>  Actually, you have to be even more careful than that; you may only
>  include not just operations that may be safely repeated, but
>  operations that may be erroneously executed.  Consider the following
>  snippet: