Warn about unusable instances when types are missing from exports #3394

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1. garyb

   added

   type: enhancement

   error-message

   on Jun 28, 2018
2. garyb

   added this to the 1.0 milestone on Jun 28, 2018
3. hdgarrood commented on Dec 17, 2018

   

   hdgarrood

   on Dec 17, 2018

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   I think I also ran into this while trying to put together a generic implementation of the Debug class in https://github.com/hdgarrood/purescript-debugged/, by exporting GenericDebug but forgetting to export GenericDebugArgs. Just to check, is this what you had in mind?

   module Main (Tuple(..), class Y, y) where
   
   data Tuple a b = Tuple a b
   
   class X a where
     x :: a
   
   instance xInt :: X Int where
     x = 0
   
   class Y a b where
     y :: Tuple a b
   
   instance yInt :: X a => Y a Int where
     y = Tuple x 1

   Then any attempt to write e.g. (y :: Tuple Int Int) in another module will give you

     No type class instance was found for class
       Main.X
     because the class was not in scope. Perhaps it was not exported.
   while solving type class constraint
     Main.X Int
   

   Have we considered having a non-exported type or class appear in an instance for an exported class be an error? It seems consistent with the existing behaviour of requiring members of an exported class to be exported, and I don't think it ever makes sense to export a class but make one or more of its instances unusable by not exporting things they reference.
4. garyb commented on Dec 17, 2018

   

   garyb

   on Dec 17, 2018

   Last edited by garyb

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   Have we considered having a non-exported type or class appear in an instance for an exported class be an error?

   Yeah, that's essentially how I'd like to fix this.

   edit: Oh, I see what you mean - I said warning, but it could be an error immediately. Yeah, that works too!