On Wed, Oct 29, 2014 at 10:39 AM, Herbert Valerio Riedel <hvr@gnu.org> wrote:

Hi Michael,

On 2014-10-29 at 00:43:55 +0100, Michael Snoyman wrote:
> I don't want to make a format proposal yet, just open up discussion on an
> issue, and see how others feel about it.
>
> As I recently commented on this list[1], the Show typeclass is overloaded
> with multiple meanings (serialization, debug info, and user-friendly data
> display). The general consensus seems to be that the official semantics for
> Show should be for serialization (as paired up with Read).

[...]

> I have a possible solution that I'd like to propose as a strawman: add a
> new method to the Exception typeclass:
>
> displayException :: e -> String
> displayException = show

As we're having the general problem of 'Show' being abused for
pretty-printing, why not rather standardise on a pretty-show/printing
typeclass that would not be limited to Exceptions? What am I missing?

The downside to that is that it introduces a breaking change, whereas this proposal does not. If we *do* want to standardize on a pretty-printing typeclass, I think the right way forward (though somewhat painful) is:

Release X: Introduce the pretty-printing typeclass, add displayException with default implementation of Show, do not otherwise change Exception.
Release X+1: Add new typeclass as a superclass of Exception.

Release X+2: Switch default implementation of displayException to the pretty-show.
Release X+3: Remove Show as a superclass.

Maybe some of those steps can be combined, I'm not certain.

Michael