My first suspicion would be simplified subsumption. Try eta-expanding them.
On Fri, May 12, 2023 at 5:10 PM Olaf Klinke
Dear Cafe,
I'm an infrequent lens user so please forgive me if the below problem has a trivial solution.
For a long time the documentation of Control.Lens.Zoom has been claiming that 'zoom' and 'magnify' can be specialized to the following types.
{-# LANGUAGE RankNTypes #-} -- Type error with GHC 9.0.2 or 9.2.7, okay with GHC 8.8.4 import Control.Lens.Zoom (zoom,magnify) import Control.Lens.Type (Lens') import Control.Monad.RWS (RWS) zoomRWS :: Monoid w => Lens' s s' -> RWS r w s' a -> RWS r w s a zoomRWS = zoom magnifyRWS :: Monoid w => Lens' r r' -> RWS r' w s a -> RWS r w s a magnifyRWS = magnify
Indeed GHC 8.8.4 compiles this happily (on lens-4.18.1), but 9.0.2 (on lens-5.0.1) throws nearly undecipherable errors, claiming that Lens' can not be matched with some more specialized type involving Magnified and LensLike. The source of Control.Lens.Zoom does not differ substantially between 4.18.1 and 5.0.1 so my guess is it is a type checker issue. Can anyone explain? On which bug tracker should I raise this issue, if it is indeed a valid one? (I pulled the above versions from stackage lts-16.31, -19.5 and -20.20, respectively and compiled with stack.)
Thanks Olaf
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-- brandon s allbery kf8nh allbery.b@gmail.com