Re: suggestion: A common type class for mutable variables

The first option is class Monad m => MonadRef r m | m -> r where newRef :: a -> m (r a) ... This has the benefit of using quite portable extensions. The second option is class Monad m => MonadRef m where type Ref m :: * -> * newRef :: a -> m (Ref m a) This takes us into GHC specific territory, by using type families, but avoids polluting every type that uses one with an extra 'ref' param. I use this variant in my as-yet-unreleased 'revisions' package. Both of these have the benefit that they can work with transformers, but they carry the limitation that you can't have multiple reference types for the same monad. e.g. you can't use the same combinators for both IORefs and, say, MVars or TVars within the same monad. This is arguably not so much a problem as they have very different operational semantics! In other code I've done horrible things for lifting TVars out into monad transformer stacks, while well aware of the rather ad hoc meaning of chains of operations on TVars when you are working over something IO based. Peeking at some haddocks from my analytics project: class Monadhttp://rebase.org/Library/Frameworks/GHC.framework/Versions/7.4.1-x86_64/usr... m => MonadSTM m whereSourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#MonadSTM stm :: STMhttp://rebase.org/Library/Frameworks/GHC.framework/Versions/7.4.1-x86_64/usr... a -> m aSourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#stm newTV :: a -> m (TVarhttp://rebase.org/Library/Frameworks/GHC.framework/Versions/7.4.1-x86_64/usr... a)Sourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#newTV readTV :: TVarhttp://rebase.org/Library/Frameworks/GHC.framework/Versions/7.4.1-x86_64/usr... a -> m aSourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#readTV newTMV :: a -> m (TMVarhttp://rebase.org/Library/Haskell/ghc-7.4.1/lib/stm-2.3/doc/html/Control-Con... a)Sourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#newTMV newEmptyTMV :: m (TMVarhttp://rebase.org/Library/Haskell/ghc-7.4.1/lib/stm-2.3/doc/html/Control-Con... a)Sourcehttp://rebase.org/analytics/src/Data-Analytics-Task-STM.html#newEmptyTMV newTC :: m (TChanhttp://rebase.org/Library/Haskell/ghc-7.4.1/lib/stm-2.3/doc/html/Control-Con... a) This had the benefit of staying in 98 style, but solved a different problem than what we're discussing here, because the reference type was fixed. In yet other code i've had the reference type select the monad it works with. The primitive package that is used by vector is at least morally of this flavor, though, for array types rather than references, but another example is the 'var' package that was released the other day in response to this thread. It doesn't use any functional dependencies between the monad and the reference type in order to support multiple reference types per monad, including unboxed variants. -Edward On Mon, Jun 3, 2013 at 2:38 PM, Petr Pudlák wrote:

Could you please just shortly comment about those two? I'd be interested to know.

Thanks, Petr

Dne 06/03/2013 08:32 PM, Edward Kmett napsal(a):

I see little hope in standardizing as, heck, I personally have felt the need to use two of the different points in the design space within just my own code.

-Edward

On Mon, Jun 3, 2013 at 2:13 PM, harry wrote:

Henning Thielemann writes:

I have started a Wiki page in order to keep an overview of all the existing approaches: http://www.haskell.org/haskellwiki/Mutable_variable

There seems to be a fair amount of duplication in all this ... any idea how a "standard" approach could be selected?

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