PROPOSAL: Add displayException to Exception typeclass

[Simon Hengel]

> 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].

Other programming language communities use `toString`, `str` and `to_s`
for this primitive [4].  So maybe ToString/toString would be a better
class/method name.

> 2. Modify GHC's default exception handler to use `displayException`
> instead of `show`.

I'm -1 on this one.  From my perspective most of the time uncaught
exceptions are encountered by programmers.  So I would prefer to have
the programmer friendly version by default.  In cases where I want the
"user friendly behavior" I'm ok with the extra step of adding a custom
exception handler.  Ideally, we would already provide a convenient way
to do so.

Cheers,
Simon

[1] http://hackage.haskell.org/package/interpolate
[2] http://hackage.haskell.org/package/interpolatedstring-qq
[3] http://hackage.haskell.org/package/interpolatedstring-perl6
[4] https://github.com/sol/exceptions-by-language