Safe Haskell | Safe-Inferred |
---|---|
Language | Haskell2010 |
Documentation
class PrimMonad m => MonadST m where Source #
This class is for abstracting over stToIO
which allows running ST
actions in IO
. In this case it is to allow running ST
actions within
another monad m
.
The normal type of stToIO
is:
stToIO :: ST RealWorld a -> IO a
We have two approaches to abstracting over this, a new and an older
(deprecated) method. The new method borrows the primitive
package's
PrimMonad
and PrimState
type family. This gives us:
stToIO :: ST (PrimState m) a -> m a
Which for IO
is exactly the same as above. For ST
it is identity, while
for IOSim
it is
stToIO :: ST s a -> IOSim s a
The older (deprecated) method is tricky because we need to not care about
both the IO
, and also the RealWorld
, and it does so avoiding mentioning
any s
type (which is what the PrimState
type family gives access to).
The solution is to write an action that is given the liftST
as an argument
and where that action itself is polymorphic in the s
parameter. This
allows us to instantiate it with RealWorld
in the IO
case, and the local
s
in a case where we are embedding into another ST
action.
stToIO :: ST (PrimState m) a -> m a Source #
Since: 1.4.1.0
withLiftST :: (forall s. (forall a. ST s a -> m a) -> b) -> b Source #