[core libraries] Data.Display - Textual representation of Haskell values (was Re: PROPOSAL: Add displayException to Exception typeclass)

[Michael Snoyman]

On Mon Nov 10 2014 at 7:17:07 PM Simon Hengel <sol at typeful.net> wrote:

> > > 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).
>
> I created an initial draft:
>
>     https://github.com/sol/display/blob/master/src/Data/Display.hs
>
>
Is this intended to be a standalone library, or part of text (or some other
package?).


> I named the builder version `displayBuilder` as I could not come up with
> any better name (suggestions much appreciated!).
>
> In addition, I think the `display` method has to be outside of the
> class.  Otherwise, if somebody defines `display` he end up with the
> default implementation of `displayBuilder` (which is based on show and
> may not be what is intended).  Any input on that?
>
>
I don't think this is a good enough reason to avoid a possible
optimization. I'd recommend documenting this aspect of the typeclass
thoroughly, and then let people shoot themselves in the foot if desired.

Michael

PS: I just saw Greg's email, I'd like to +1 his questions, clarification is
good.
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