[Haskell-cafe] Representation of lenses

[David Turner]

Thanks, I'll give that a watch.
On 27 Jan 2015 17:23, "Charles Durham" <ratzes at gmail.com> wrote:

> You need to sign up for it, but this is a phenomenal talk by simon peyton
> jones describing the idea behind lenses the way he understood it.
>
>
> https://skillsmatter.com/skillscasts/4251-lenses-compositional-data-access-and-manipulation
>
> On Tue, Jan 27, 2015 at 12:18 PM, David Turner <
> dct25-561bs at mythic-beasts.com> wrote:
>
>> Hi,
>>
>> I'm planning on discussing lenses with some colleagues shortly so I'm
>> consolidating some of my notes into a coherent story. At one point I
>> found myself describing lenses as a way of packaging up a getter and a
>> setter into a single thing. However, this raises the question: why not
>> just use a pair (getter, setter)? More precisely, I believe that the
>> types (a -> s, s -> a -> a) and (forall f. Functor f => (s -> f s) ->
>> a -> f a) are isomorphic. Is that right?
>>
>> I see that one advantage of the lens type is that you can use (.) to
>> compose them since they're just functions, but that doesn't bother me
>> much and it seems I could define another operator to compose (getter,
>> setter) lenses and the rest of the machinery would work either way.
>>
>> It's also possible that you can't get the full generality of (forall
>> f. (s -> f t) -> a -> f b) lenses with getter/setter pairs, although I
>> haven't worked through the details yet so don't know either way.
>>
>> So, my question is: what is the advantage of representing lenses in
>> the way that they are?
>>
>> Many thanks,
>>
>> David
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>
>
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