relaxing instance declarations
doug at cs.dartmouth.edu
Wed May 1 01:05:32 CEST 2013
Max's idea (see below) of a second where clause is cute, but
not sanctioned by Haskell syntax.
Iavor wrote, "It would be quite arbitrary to restrict this only
Actually what I have in mind is to make the language MORE
consistent, by eliminating distinctions between instance-wheres
and ordinary declaration-wheres. Currently instance-wheres may
only declare class methods, while declaration-wheres may declare
variables at will. Also instance-wheres may not declare type
signatures, while declaration-wheres may. I propose dropping
these restrictions on instance-wheres.
Hazard: Adding a method to an existing class could accidentally
capture a name that was previously local to an instance-where.
Capture can be prevented by declaring type signatures for local
variables. The compiler might warn when such defensive
declarations are lacking.
On Mon, 29 Apr 2013 15:56 Iavor Diatchki <iavor.diatchki at gmail.com> wrote
I think that if we want something along those lines, we should consider a
more general construct that allows declarations to scope over other
declarations (like SML's `local` construct). It would be quite arbitrary
to restrict this only to instances.
On Mon, Apr 29, 2013 at 2:41 PM, Max Bolingbroke <batterseapower at hotmail.com
> You could probably get away with just using two "where" clauses:
> instance Foo a where
> bar = ...
> auxilliary = ...
> On 28 April 2013 18:42, Edward Kmett <ekmett at gmail.com> wrote:
>> Makes sense. I'm not sure what a good syntactic story would be for that
>> feature though. Just writing down member names that aren't in the class
>> seems to be too brittle and error prone, and new keywords seems uglier than
>> the current situation.
>> Sent from my iPad
>> On Apr 28, 2013, at 1:24 PM, Doug McIlroy <doug at cs.dartmouth.edu> wrote:
>> > Not always. For example, you can't mess with the declaration
>> > of a standard class, such as Num.
>> > On Sun, Apr 28, 2013 at 12:06 PM, Edward Kmett <ekmett at gmail.com>
>> >> You can always put those helper functions in the class and then just
>> >> export them from the module.
>> > On Sun, Apr 28, 2013 at 10:49 AM, Doug McIlroy <doug at cs.dartmouth.edu
>> >> Is there any strong reason why the where clause in an instance
>> >> declaration cannot declare anything other than class
>> >> operators? If not, I suggest relaxing the restriction.
>> >> It is not unusual for declarations of class operators to
>> >> refer to special auxiliary functions. Under current rules
>> >> such functions have to be declared outside the scope in
>> >> which they are used.
>> >> Doug McIlroy
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