On Fri, Aug 3, 2012 at 10:11 AM, Jonathan Geddes
The nice part about the SEC functions is that they compose as regular functions. Lenses are super powerful in that they form a category. Unfortunately using categories other than functions feels a tad unwieldy because you have to hide something from prelude and then import Category. (A bit like exceptions, currently).
FWIW this is also true for van Laarhoven lenses[1] type FTLens a b = forall f. Functor f => (b -> f b) -> (a -> f a) newtype Const a b = Const { unConst :: a } deriving Functor get :: FTLens a b -> a -> b get ft = unConst . ft Const {- ft :: forall f. (b -> f b) -> (a -> f a) Const :: forall x. b -> Const b x ft Const :: a -> Const b a -} newtype Id a = Id { unId :: a } deriving Functor set :: FTLens a b -> b -> a -> a set ft b = unId . ft (\_ -> Id b) modify :: FTLens a b -> (b -> b) -> a -> a modify ft k = unId . ft (Id . k) -- example fstLens :: FTLens (a,b) a fstLens aToFa (a,b) = (,b) <$> aToFa a -- and you get compose :: FTLens b c -> FTLens a b -> FTLens a c compose = (.) identity :: FTLens a a identity = id
If you like the look of "set" with lenses, you could define a helper function to use with SEC updaters.
set :: ((b -> a) -> c) -> a -> c set sec = sec . const
--and then use it like so: setPersonsSalary :: Salary -> Person -> Person setPersonsSalary salary = set personsSalary' salary
With it you can use an updater as a setter. I'd like to reiterate one of finer points of the original proposal.
The compiler could disallow using old-style update syntax for fields whose SEC update function is not in scope, giving us fine-grained control over access and update. On the other hand we currently have to create new functions to achieve this (exporting the getter means exporting the ability to update [using update syntax] as well, currently).
And now back to lenses:
it is really convenient how lenses let you compose the getter and setter together.
I don't recall too many cases where having the getter and setter and modifier all in one place was terribly useful. Could anyone give me an example? But again, where that is useful, a lens can be created from a getter and a SEC updater.
Thoughts?
--Jonathan
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