Add deeply strict liftM

[treeowl]

Control.Monad offers <$!>. By analogy, I believe this package should offer

(<$!!>) :: (Monad m, NFData b) => (a -> b) -> m a -> m b
f <$!!> m = m >>= \x -> return $!! f x

[RyanGlScott]

Sanity-check: is this quite analogous to (<$!>) from base? Its definition is:

(<$!>) :: Monad m => (a -> b) -> m a -> m b
f <$!> m = do
  x <- m
  let z = f x
  z `seq` return z

Whereas with (<$!!>), we have something that's closer to:

(<$!!>) :: (Monad m, NFData b) => (a -> b) -> m a -> m b
f <$!!> m = do
  x <- m
  f x `deepseq` return (f x)

I'd hope that GHC is smart enough to apply CSE on the two occurrences of f x, but maybe it'd be wise to use explicit sharing here.

[treeowl]

I don't understand how it could translate the way you say. Doesn't $!! take care of the sharing here? If not, that's broken.

[treeowl]

Actually, $!! shouldn't have to worry about it either. Unless GHC actually copies the expression for no reason, we should be fine.

[RyanGlScott]

Ah, you're right, I inlined one too many definitions in my head. The fact that you're using ($!!) here explicitly takes care of that issue. Carry on :)

[treeowl]

The bigger problem is that <$!> is weird. It should really be called <!$>. <$!> should force the argument; <!$!> should force both. For deepseq, I think only <!!$> and <$!!> are really useful. But it's too late to fix this, so we should follow base for consistency.