On Sat, Oct 16, 2010 at 21:39, Stephen Tetley
Hello list
The Monad and Applicative instances for functions are "equivalent" to the respective Reader vesions (I use "equivalent" along the lines of - operationally the same but without the type distinction / newtype). There is also the Monoid instance for functions which is pretty slick.
Has anyone looked at variations with two or more static arguments though?
For example mappend with two static arguments needs to be defined either as a new function:
mappendR2 :: Monoid a => (r1 -> r2 -> a) -> (r1 -> r2 -> a) -> r1 -> r2 -> a mappendR2 f g = \x y -> f x y `mappend` g x y
or an overlapping instance:
instance Monoid a => OPlus (r1 -> r2 -> a) where f `mappend` g = \x y -> f x y `mappend` g x y
I think you can just use the original instance for Monoid for this. It declares instance Monoid b => Monoid a -> b since 'b' can itself be a function (and r1 -> r2 -> a is parenthesized as r1 -> (r2 -> a)), this will work. 'b' can be instantiated to (r2 -> a) in your case, which is a Monoid due to another instance for functions, this time with 'b' instantiated to 'a'. This doesn't work for Reader, though. There, you're probably easier off using just a tuple, or, even better, a record, where you can use the 'asks' function to extract a single component. Erik