Re: [Haskell-cafe] Why Haskell?

29 Mar 2021

      Thanks for the insights, I was not aware of this approach before. 

But with STM as the base monad, I feel like to have lost the ability to delimit transaction boundaries at will, which is the essential tool STM offers. I can only think of doing that from IO via `atomically`, but that way we are dragged back to write transaction composing code in IO, so will lose effect tracking.

I dunno but is there a way to delimit STM transactions without IO?
...
On 2021-03-29, at 19:50, Viktor Dukhovni  wrote:
STM is not a monad transformer, but it is a fine base monad, just like
Identity, IO or ST.  Here's a contrived example of (StateT Int STM Int):
import Control.Concurrent.STM
   import Control.Monad (when)
   import Control.Monad.Trans.State.Strict
   import Control.Monad.Trans.Class (lift)
--
   main :: IO ()
   main = do
      tv <- newTVarIO 0
      y <- atomically $ flip evalStateT 0 $ do
          x <- get
          lift $ do
              modifyTVar tv (\a -> a + x + 1)
              y <- readTVar tv
              when (y > 10) retry
              return y
      print y
So any or all of RWST work with STM, but you typically want to keep your
STM transactions small and simple, so this is not a place where one
would generally run wild with fancy stacks that do non-trivial
additional computation.
Indeed Monad Transformers are not Monads, they're always stacked on top
of some base monad.  The turtles don't go all the way down.
A practical example of STM-like Monad's can be found in Hasql, where
database operations run in a Monad that ensures that they have no
side-effects that would prevent the transaction from being retried on
deadlock detection.  This is also a base Monad, where if you like
you can stack more (pure) transformers.
Which reminds me that ExceptT can be useful in such Monads, which
avoid throwing impure exceptions.  And is used in Hasql, where
the operations tend to be more expensive than in STM, and any
overhead from layering ExceptT or similar is quite small.
-- 
   Viktor.
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Re: [Haskell-cafe] Why Haskell?

YueCompl