Re: [Haskell-cafe] primitive operations in StateT

On 16 April 2013 15:56, Clark Gaebel wrote:

The monad my code is currently written in is:

type MC = MCT Identity -- where MCT is the monad transformer version of it.

I have two options for threading state through this:

MCT (ST s) a StateT s MC a

The first option would work if I had some function with the signature

MCT Identity a -> MCT (ST s) a

but I know of no such function, and the second one would work if I had some way of making StateT a member of PrimMonad.

Can I see an example with 'lift'?

You should just be able to use lift to convert any PrimMonad action into your MCT-based monad as long as MCT is an instance of the MonadTrans class. i.e. if `foo :: (PrimMonad m) => m a' and MCT is an instance of MonadTrans, then you can do `lift foo :: (PrimMonad m) => MCT m a', or `lift (lift foo) :: (PrimMonad m) => MCT (StateT s m) a'. As another option, I have an as-yet-unreleased library to let you avoid having to do lift . lift for PrimMonad-based actions: http://hub.darcs.net/ivanm/monad-prim

- Clark

On Tuesday, April 16, 2013, Ivan Lazar Miljenovic wrote:

...

On 16 April 2013 15:04, Clark Gaebel wrote:

...

Hi list!

I want to use MVectors in a StateT monad transformer.

How do I do that? StateT isn't a member of 'PrimMonad', and I have no idea how to make it one.

You can use Control.Monad.Trans.lift to lift the PrimMonad operations to PrimMonad m => StateT s m

...

Regards, - Clark

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-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com