Should primMonad class live in base or mtl?

David Feuer david.feuer at
Tue Jun 16 02:03:13 UTC 2020

I'm opposed to moving it to base. This locks in base to GHC's fundamentally
broken model of IO (in terms of functions passing state tokens) and also
ties together the fundamentally different concepts of IO and ST. These
issues tend to crop up in strictness analysis, and GHC has to continually
tweak various internal hacks to keep users from noticing (most of the
time). Let's not make future Haskells make these same decisions.

On Mon, Jun 8, 2020, 9:18 AM Carter Schonwald <carter.schonwald at>

> This is something I’ve pondered on and off, And the classes PrimMonad and
> PrimMonadBase are super stable
> Argument in favor of base :
> there’s absolutely code in base, and packages like array, which would
> benefit from PrimMonad.   Plus it might allow more reuse / encourage more
> ST monad code in the wild? (I always feel like ST monad is underused. )
> Argument against base:
> Then a lot of PrimMonad instances would need to live in transformers. And
> I’m not sure if maintainers over there would want that?
> Argument in favor of mtl: it’s where all the other transformer stack stuff
> gets defined. So all the upkeep for monad instances is already going on
> there regularly.
> Argument against mtl :  aside from concentrating the upkeep on transformer
> stack instances it seems like it wouldn’t enable new improvements to base.
> Another option is to have some sort of clone of the class in base , as
> long as it pays for itself in code reuse / simplification within base
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