Re: Proposal: merge either into transformers

On Sun, Apr 27, 2014 at 10:47:56AM +0300, Michael Snoyman wrote:

On Sat, Apr 26, 2014 at 9:57 PM, Ross Paterson wrote:

...

would be incompatible with the lazy WriterT in subtle ways

That would be troubling, but I'm not sure in which ways it's incompatible. Do you have any examples?

The Applicative and Alternative instances would have different contexts, and there would be no instances for Foldable, Traversable, Eq, Ord, Read or Show.

The advantage of having WriterT implemented in terms of strict StateT is that many people will automatically get the fix when upgrading to transformers 0.4. Also, the writer API itself is very convenient for many common use cases, so it would be nice if there was a version available that didn't leak memory.

Invisibly increasing strictness cuts both ways: some people get space savings, while others get non-termination. (Although one might say they're not using it properly, that seems a harsh punishment.) But I'm more concerned about the complication. Currently the transformers are transparent, and people can think about the type structure of the monad they're building. This very transformer is already there; it seems simpler to direct people to it instead of having another copy. The API point is an issue, though.