Why are there no Show instances for internal types

[Simon Peyton Jones]

| > >- Is there a reason that there are no derived 'Show' instances for
| > >most types?
| >
| > As Richard mentioned, we don't derive Show due to code size and
| compilation time concerns.

| > Show in particular is rather expensive to derive and seeing as we
| already have Outputable I don't it would make sense to derive it by
| default.
| 
| Show and Outputable have very different goals though.

Really? What's wrong with using Outputable, plus, as Joachim says, 

	showSDocUnsafe . ppr :: Outputable a => a -> String

Maybe you want to really really see the precise data constructors used.  But for the most part the Outputable instance tells you that, but much more legibly.

Simon