Why are there no Show instances for internal types

[Edward Z. Yang]

We can't add Show instances for these types because many types
below them, e.g., Type, are cyclic, and would result in infinite
output.

Perhaps we can add a new type class which a) faithfully represents
the Haskell syntax, but b) can deal with cyclic data.  I think that's
something people would like (extra compilation time not withstanding).
But it sounds annoying to do since the deriving mechanism is not going
to help you.

Edward

Excerpts from Tom Sydney Kerckhove's message of 2017-03-18 14:03:48 +0100:
> Dear GHC Devs,
> 
> I am trying to use GHC as a library but I'm having a lot of trouble with
> understanding what everything means.
> Up to now, I have been able to figure out what to do by reading the
> sources, but it ocured to me that much of my struggles could have been
> mitigated if the relevant types had Show instances.
> 
> I am specifically talking about the types concerning type checking.
> TypecheckedModule and everything below that.
> I am aware that most of the types have an Outputable instance, but
> there are two problems with that:
> 
> - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> - These instances are not intended to show the internal structure of a
>   value, but rather a 'human readable' representation of a value.
> 
> My questions for you:
> 
> - Is there a reason that there are no derived 'Show' instances for most
>   types?
> - Would you accept a diff that adds these?
> 
> Thank you for your time.
>