[Haskell-cafe] type constructor section for (-> Bool), _not_ ((->) Bool)

[AntC]

> Brent Yorgey <byorgey <at> seas.upenn.edu> writes:
> 
> > On Tue, Sep 03, 2013 at 11:33:46AM +0000, AntC wrote:
> > 
> > I want an instance and type improvement constraint of the form
> > 
> >     instance (f ~ (-> Bool)) => C Foo (f b) where ...
> > 
> 
> There is no operator section syntax for types.  Moreover, this is not
> just an issue of syntax: it is impossible to partially apply a type
> constructor to anything other than its first argument, because there
> is no type-level lambda.
> 

Thank you Brent, so I'm not being entirely dumb ;-).

> > 
> >     data FlipFun b          -- abstract
> >     instance (f ~ FlipFun) => C Foo (f b) where ...
> > 
> > And a type function inside the class to generate the type.
> > But then I'd have to apply the type function for all instances,
> > and in most places it'd be id.
> 
> That's the only way to do it that I know of.
> 

OK, I've got that working. My class is a helper doing type discrimination. 
So the constraints have 'infected' the caller and the caller's caller, 
etc ...

Elegant it ain't.

AntC