default roles

[Joachim Breitner]

Hi,

Am Mittwoch, den 09.10.2013, 15:21 -0400 schrieb Richard Eisenberg:
> Wait! I have an idea!
> The way I've been describing GND all along has been an abbreviation.
> GHC does not coerce a dictionary from, say, Ord Int to Ord Age.
> Instead, GHC mints a fresh dictionary for Ord Age where all the
> methods are implemented as coerced versions of the methods for Ord
> Int. (I'm not sure why it's implemented this way, which is why I've
> elided this detail in just about every conversation on the topic.)
> With this in mind, I have a proposal:
> 
> 
> 1) All parameters of all classes have nominal role.
> 2) Classes also store one extra bit per parameter, saying whether all
> uses of that parameter are representational. Essentially, this bit
> says whether that parameter is suitable for GND. (Currently, we could
> just store for the last parameter, but we can imagine extensions to
> the GND mechanism for other parameters.)
> 
> 
> Because GND is implemented using coercions on each piece instead of
> wholesale, the nominal roles on classes won't get in the way of proper
> use of GND. An experiment (see below for details) also confirms that
> even superclasses work well with this idea -- the superclasses aren't
> coerced.

what do you need the extra bit for? During GHD, can?t you just create
the new dictionary (using "method = coerce original_method) and then see
if it typechecks, i.e. if the method types can be coerced.

(If not, the error messages might need massaging, though.)

Greetings,
Joachim

-- 
Joachim ?nomeata? Breitner
  mail at joachim-breitner.de ? http://www.joachim-breitner.de/
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  Debian Developer: nomeata at debian.org
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