Provide unfoldM-style generalizations of fromList

[hvr]

This has been suggested by @tibbe:

fromList and fromListWith are optimized to use contained in-place mutation via unsafeInsert(With).

If the data to be inserted requires side-effects (like e.g. reading lines from /dev/stdin w/o resorting to lazy I/O), an unfoldM-style generalization might be useful.

(somewhat related: haskell/vector#8)

[sjakobi]

Also related, if I understand the issue correctly: haskell/containers#405

[treeowl]

Tricky. This can't use an arbitrary Monad. It's more a PrimBase sort of thing, and rather unsafe in the presence of broken instances of that.

[sjakobi]

Probably also related: #25.

[sjakobi]

What's the type that this thing should have? Something like this?

unfoldM :: (PrimMonad m, Hashable k) => (a -> m (Maybe (k, v))) -> a -> m (HashMap k v)

In vector there's this now:

unfoldrM :: (Monad m) => (b -> m (Maybe (a, b))) -> b -> m (Vector a)
unfoldrNM :: (Monad m) => Int -> (b -> m (Maybe (a, b))) -> b -> m (Vector a)
unfoldrExactNM :: (Monad m) => Int -> (b -> m (a, b)) -> b -> m (Vector a)

[sjakobi]

Regarding the PrimMonad constraint, this should be viable once u-c depends on primitive: #197