PROPOSAL: Add displayException to Exception typeclass

[Simon Hengel]

On Mon, Nov 10, 2014 at 10:49:44AM -0500, Roman Cheplyaka wrote:
> On 10/11/14 09:56, Simon Hengel wrote:
> >> 1.  Add a new method to the Exception typeclass:
> >>
> >>     -- | Render this exception value in a human-friendly manner. Default
> >> implementation: @show at .
> >>     displayException :: e -> String
> >>     displayException = show
> > 
> > I'm +0.5 on this one, even though I think we should solve this in the
> > general case.  I think there is a difference between converting
> > something to a string (display) vs. showing something in a programmer
> > friendly way (show).  This distinction is not novel, both Python and
> > Ruby make that distinction (str()/repr() in Python, #to_s/#inspect in
> > Ruby).
> > 
> > So I would love to have tho following type class:
> > 
> >     class Display a where
> >       display :: a -> String
> >       default display :: Show a => a -> String
> >       display = show
> > 
> > Note that for many Prelude types `show == display`, with the notable
> > exception of String, where `display = id`.  One use case where this
> > matters is string interpolation [1][2][3].
> 
> I would also like a class like that to exist.
> 
> But I think it should be based on Text and Text builder rather than
> String (and hence be outside of base).

Yes, this is a valid point.  The Text vs. String issue is one of the
things that made me reluctant to tackle this so far (the other being
that outside of base people may not want to depend on it, orphan
instances, ...).  My given Display class would probably not be that
useful without some `displays :: ShowS`.

I think by now I'm sold on the separate package based on text.

Cheers,
Simon