STM check/MonadPlus

[Simon Peyton-Jones]

Thanks for the typo.  Yes, for Haskell guys 'guard' is fine; but the
main audience for the paper is non-haskell folk, so we have to spell out
the defn.

S

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Remi Turk
| Sent: 06 February 2005 20:25
| To: glasgow-haskell-users at haskell.org
| Subject: STM check/MonadPlus
| 
| Hi,
| 
| I have a few questions about the `check' function from
| Control.Concurrent.STM.
| 
| First, in the paper its definition contains a (type) error:
| 
| check :: Bool -> STM a
| check True  = return ()
| check False = retry
| 
| in fptools, however, it is defined as follows:
| 
| check :: Bool -> STM a
| check b = if b then return undefined else retry
| 
| As we already have guard, and STM is an instance of MonadPlus,
| I wonder why such a simple utility function as check is wanted
| at all.
| 
| guard           :: (MonadPlus m) => Bool -> m ()
| guard True      =  return ()
| guard False     =  mzero
| 
| Greetings,
| Remi
| 
| --
| Nobody can be exactly like me. Even I have trouble doing it.
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