[Haskell-beginners] Transforming (State s a) to (StateT s m a)

Yitzchak Gale gale at sefer.org
Tue Aug 4 09:09:32 EDT 2009

Hi Toby,

Toby Murray wrote:
> Suppose I have a transformer A of type State s a...
> Suppose I have other state transformers of type StateT s m a...
> I can manually convert A to be of the second kind as follows...
> pure :: (Monad m) => State s a -> StateT s m a
> pure func = StateT (\arg -> (return (runState func arg)))

Yes, this does come up from time to time. It could be
useful to give it a standard name in the libraries.

But "pure" is not a good name - that is firmly entrenched
to mean other important things in Control.Applicative
and Control.Arrow.

Perhaps liftT?

liftT :: (Monad m) => State s a -> StateT s m a
liftT (State f) = StateT $ return . f

You can submit your proposal by following this



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