Why are there no Show instances for internal types

[Tom Sydney Kerckhove]

On 19-03-17 02:08:56, Rahul Muttineni wrote:
> Syd, can you tell us what kind of things you were trying to print out?

Maybe I wasn't very clear.
I'm trying to visualise the internal structure of some of the
typechecker's output.
I specifically do NOT need to see the output of Outputable's functions.
They show the human-readibly version and not the internal structure.

Does that answer your question?
> Hope that helps,
> Rahul
> 
> On Sun, Mar 19, 2017 at 1:14 AM, 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.
> > >
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >
> 
> 
> 
> -- 
> Rahul Muttineni

-- 
Tom Sydney Kerckhove
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