Export runListT?

[kozak]

Should runListT be exported? I stumbled on this when going through this:
https://github.com/Gabriel439/Haskell-List-Transformer-Library/blob/master/src/List/Transformer.hs
where runListT is used.

Ok it seems its a different runListT - so it probably shouldn't be exported.

[matthewleon]

It does seem bizarre that the library defines runListT and then seems to do nothing with it.

[paf31]

It should be removed since ListT has a Newtype instance now.

[garyb]

It should be removed since ListT has a Newtype instance now.

It looks like there is a bug here... ListT's constructor is not exported, nor is Step, the inner type, so I don't think that Newtype instance should be allowed.

[matthewleon]

Is the correct approach to export just the constructor and remove the Newtype instance? That would be breaking.

[garyb]

We'll be doing some breaking updates soon anyway... but there is a non-breaking solution too: export the stuff that is hidden!

I think I asked @paf31 about it before and he thought that it was better to keep it private though.

[matthewleon]

At the very least, for consistency with the other transformers, it seems to me that runListT should indeed be exported. I imagine that @paf31 has some good reason for keeping it this way?

As for not exposing the ListT constructor, that makes some sense, as nil, singleton, unfold etc. provide ample ways to enter the monad.

[matthewleon]

(adding to the above, perhaps what we really want is some modified version of runListT that "unrolls" the monad into a list, strict or lazy, in the parent monad?)

[matthewleon]

https://hackage.haskell.org/package/list-t-1/docs/ListT.html

For reference, the above package provides such facilities as toList.

[paf31]

I think I asked @paf31 about it before and he thought that it was better to keep it private though.

I think it's probably fine, especially since we have a Newtype instance anyway now. Let's just export the constructor.

[kozak]

In the Gabriel's library runListT :: Monad m => ListT m a -> m () is described as 'Use this to drain a ListT, running it to completion and discarding all values. And I think something like this should work:

runListT' :: forall m a. (Monad m) => ListT m a -> m Unit
runListT' = foldl const unit

?

[paf31]

I think you would need foldl' at least, but there's no reason we couldn't provide that function.