Stricter WriterT (Part II)

Edward Kmett ekmett at
Wed Mar 20 01:33:37 CET 2013

I use them fairly heavily, mostly so I can make packages that work with
arbitrary transformer stacks.

I'd be willing to endure the pain of using #if
MIN_VERSION_transformers(x,y,z) bracketing around the use of Strict.WriterT
and Strict.RWST constructors so long as there were writerT/runWriterT and
rwsT/runRWST analogues I could simply invoke.


On Tue, Mar 19, 2013 at 11:07 AM, Gabriel Gonzalez <gabriel439 at>wrote:

> Presumably we'll also need
>> writerT :: m (a, w) -> WriterT w m a
> Yes, we would need that, too:
> writerT m = WriterT $ \w -> do
>     (a, w') <- m
>     let wt = mappend w w'
>     wt `seq` return (a, wt)
>  Is there any reason to keep Control.Monad.Trans.Writer.**Strict, or
>> should this replace it?
> If we replace the old one, it will break existing code that used the
> `WriterT` constructor.  Same thing for the strict RWST constructor if we
> similarly modify that.  However, I don't know exactly how many packages use
> those constructors.  I will try to do a text search of Hackage this coming
> weekend to check and see if it is feasible to ask downstream packages that
> use `WriterT`/`RWST` constructors to set upper bounds on `transformers`.
> ______________________________**_________________
> Libraries mailing list
> Libraries at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Libraries mailing list