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

Brent Yorgey byorgey at seas.upenn.edu
Tue Sep 3 19:12:30 CEST 2013

On Tue, Sep 03, 2013 at 11:33:46AM +0000, AntC wrote:
> I'm probably being dumb, but Hoogle nor the wiki are helping me.
> I want an instance and type improvement constraint of the form
>     instance (f ~ (-> Bool)) => C Foo (f b) where ...
> The first arg to C is driving type improvement, for example:
>     instance (f ~ []) => C Bar (f b) where ...
> (The real instances are more complex, and involve overlap, of course.)
> I'm trying to write a section to get the improved type (b -> Bool), 
> but (-> Bool) or ((-> Bool) b) is invalid syntax.

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.

> This is valid, but wrong: ((->) Bool) b  -- gives (Bool -> b).
> I could do:
>     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.


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