Build failure with transformers-0.5.6.2

[snoyberg]

Building library for refined-0.3.0.0..
[1 of 5] Compiling Refined.These    ( library/Refined/These.hs, dist/build/Refined/These.o )
[2 of 5] Compiling Refined.Internal ( library/Refined/Internal.hs, dist/build/Refined/Internal.o )

library/Refined/Internal.hs:786:29: error:
    • Couldn't match representation of type ‘Identity
                                               (Either RefineException a)’
                               with that of ‘m0 (Either RefineException a)’
        arising from a use of ‘coerce’
      The data constructor ‘Data.Functor.Identity.Identity’
        of newtype ‘Identity’ is not in scope
    • In the second argument of ‘(.>)’, namely ‘coerce’
      In the expression: ExceptT.except .> coerce
      In an equation for ‘refineM’: refineM = ExceptT.except .> coerce
    • Relevant bindings include
        refineM :: Either RefineException a -> RefineM a
          (bound at library/Refined/Internal.hs:786:1)
    |
786 | refineM = ExceptT.except .> coerce
    |        

[chessai]

Sorry for the delay, I've had an extremely busy Q1.

This is strange. Is that constructor no longer in scope? Minour version releases shouldn't cause breakage like that.

[snoyberg]

I'm not familiar with the changes in transformers, but it does seem like this was a breaking change.

[brandonhamilton]

The change made to transformers was to generalize the except function:

- except :: Either e a -> Except e a
- except m = ExceptT (Identity m)

+ except :: (Monad m) => Either e a -> ExceptT e m a
+ except m = ExceptT (return m)

which breaks the usage of coerce in

refineM :: Either RefineException a -> RefineM a
refineM = ExceptT.except .> coerce

Using visible type application fixes this with the new version:

refineM :: Either RefineException a -> RefineM a
refineM = ExceptT.except @Identity .> coerce

I'm not sure what the best way to implement this would be so that it works with both the old and newer versions of the transformers package.

[chessai]

I just added an explicit type signature to coerce: coerce :: ExceptT RefineException Identity a -> RefineM a, and checked that it built with the newest transformers as well as previous versions.