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.
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.
??
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