[Haskell] Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video

[Simon Peyton-Jones]

| In many useful cases, such as the getLine example, the Y action will have its
| own atomic {} block.  In which case the semantics of when it is allowed to
| re-attempt X are what is important.  If you require (Y) to complete before
| re-attempting (X) then you get an infinite regression where every (atomic block)
| fails with (retryWith (next atomic block)), and nothing is ever re-attempted.
| This is why "retryWith Y" meaning rollback X and do "Y >> atomic X" is the wrong
| implementation.

I don't agree.  I think it's quite reasonable.  Not many atomic blocks will finish with retryWith.  Of course there is a possibility of an infinite loop, but we already have that: f x = f x.  Of course, Y can always choose to do a forkIO, but it shouldn't hav to.

For me the only difficulty is the implementation.  We'd like to block X on the TVars it read (as usual), *unless* executing Y wrote to any of them.  That requires a bit more cleverness in the commit code, but not a great deal I think.

Simon