[Haskell-cafe] What Haskell Records Need
Ryan Ingram
ryani.spam at gmail.com
Fri Aug 3 22:57:39 CEST 2012
Oops, forgot my references
[1] Original post:
http://www.twanvl.nl/blog/haskell/cps-functional-references
[2] polymorphic update support: http://r6.ca/blog/20120623T104901Z.html
[3] another post about these:
http://comonad.com/reader/2012/mirrored-lenses/
On Fri, Aug 3, 2012 at 1:53 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
>
>
> On Fri, Aug 3, 2012 at 10:11 AM, Jonathan Geddes <
> geddes.jonathan at gmail.com> wrote:
>
>> 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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>> Haskell-Cafe at haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>>
>
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