The Coq mailing list

[Tom Prince <tom.prince AT ualberta.net>]

On Thu, 24 Nov 2011 20:16:52 -0800, Michael Shulman 
<mshulman AT ucsd.edu>
 wrote:
> Is it possible to declare a typeclass method as a hint?  When I say
> 
> Hint Resolve some_method.
> 
> if no instances have yet been declared, I get a "cannot infer implicit
> parameter" error.  If some instances have been declared, then
> apparently only the method applied to the first such instance is
> declared as a hint.  I can specify which instance I want with
> 
> Hint Resolve (@some_method some_instance).
> 
> but I want the hint to apply to *all* instances.  However, if I write
> 
> Hint Resolve @some_method.
You probably want to add ': typeclass_instances' here.
> 
> I am told "some_method cannot be used as a hint".  Is there a way to
> do this?

Is is that some_method cannot be used as a hint by *auto*? Certain hints
will only be used by eauto (and typeclasses eauto, which is what is used for
typeclass resolution).

There are a number of other things one can do. The simplest method is to use
':>' instead of ':' when declaring the projection ... this automatically
declares a hint for the projection.

You can also add arbitrary tactics as hints like
Hint Extern 4 (<some-optional-patern>) => <some-tactic> : typeclass_instances.
I have occasionally found it useful to add 'apply <something>' as an
external tactic, because this ignores typeclass transparency
... although I haven't worked out exactly why it is useful.