Why are there no Show instances for internal types

[Alan & Kim Zimmerman]

And I guess it would be bad to use Show, but make custom instances for the
problematic types that did not loop?

Alan

On 18 Mar 2017 9:44 pm, "Edward Z. Yang" <ezyang at mit.edu> wrote:

> 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.
> >
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