[Haskell-cafe] Several default implementations for instance methods

[Carter Schonwald]

hrm, so youre wanting something even smarter than the MINIMAL pragma stuff,
namely
"depending on which subset of the complementary methods are defined, define
this method differently"?

On Sat, Oct 4, 2014 at 11:44 AM, Andreas Abel <abela at chalmers.se> wrote:

> Consider the following class for an overloaded pretty printer.  For
> atomic data (numeric types, String etc.) one would implement
> prettyShow, for complex data either pretty or prettyPrec.
>
> > import Text.PrettyPrint
> >
> > class Pretty a where
> >   pretty      :: a -> Doc
> >   prettyShow  :: a -> String
> >   prettyPrec  :: Int -> a -> Doc
>
> Implementing one of these methods should be enough, giving default
> implementations for the other two.
>
> It is easy to get prettyShow and prettyPrec from pretty.
>
> >   prettyShow = render . pretty
> >   prettyPrec = const . pretty
>
> However, to define pretty from one of the others, I need two default
> implementations.
>
> >   pretty = text . prettyShow
> >   pretty = prettyPrec 0
>
> Is there a way to get this to work?
>
> Workarounds (not entirely satisfactory): Technically, one could define
> a cycle of default implementations.  Alternative 1:
>
> >   pretty       = prettyPrec 0
> >   prettyShow   = render . pretty
> >   prettyPrec _ = text . prettyShow
>
> Problem: Here, if pretty is defined,
>
> >   prettyPrec _ = text . render . pretty
>
> instead of just
>
> >   prettyPrec _ = pretty
>
> and (text . render) is not the identity (destroys inner document
> structure).
>
> Alternative 2:
>
> >    pretty       = text . prettyShow
> >    prettyShow   = render . prettyPrec 0
> >    prettyPrec _ = pretty
>
> Problem: Here, if prettyPrec is defined,
>
> >    pretty = text . render . prettyPrec 0
>
> instead of just
>
> >    pretty = prettyPrec 0
>
> I guess alternative 2 is worse than alternative 1, as one would
> usually define prettyPrec to get pretty, and not the otherway round.
> But none of these two alternatives really does the job.
>
> --
> Andreas Abel  <><      Du bist der geliebte Mensch.
>
> Department of Computer Science and Engineering
> Chalmers and Gothenburg University, Sweden
>
> andreas.abel at gu.se
> http://www2.tcs.ifi.lmu.de/~abel/
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