[Haskell-cafe] Lens, with ability to create a data-field when it's missing

[Takayuki Muranushi]

Dear everyone,

With help of many, I found ways to implement most of the features I
have wanted, and I can release the new dynamic-object package.
http://hackage.haskell.org/package/dynamic-object

Thank you in advance for your supports,

Takayuki

2013/1/3 Takayuki Muranushi <muranushi at gmail.com>:
> Dear All, I really am enjoying the lens (it is tough to learn, though :)
>
> Recently, I've been trying to implement a dynamic object --- a
> collection of methods that you can update at run-time --- using lens
> [1][2][3]. After several attempts, I think I have located the center
> of my problem:
>
> Lens with the ability "to create a data-field when it's missing." My
> question: is it possible?
>
> There are many excellent examples of partial lenses in the lens
> library, such as '_head' from Data.List.Lens 'dynamic' from
> Data.Dynamic.Lens . To me, they seem to share the following set of
> common behavior (allow me to use inaccurate terms...)
>
> - set l a s : if the field is present, insert 'a'; otherwise, do nothing.
> - over l f s : if the field is present, map 'f' over it. otherwise, do nothing.
> - preview l s : if the field is present, return 'Just a'. otherwise,
> return 'Nothing' .
>
> Instead, I want the following set of behaviors:
>
> - set l a s : if the field is present, insert 'a'; otherwise, *create
> the field* and insert 'a'.
> - over l f s : if the field is present, map 'f' over it. otherwise, do nothing.
> - preview l s : if the field is present, return 'Just a'. otherwise,
> return 'Nothing' .
>
> For example, the current behavior of _head is
>     >>> [] & _head .~ 1
>     []
> I want to implement a variant _head' , such that
>     >>> [] & _head' .~ 1
>     [1]
>
>
> Let us call this behavior 'Member' for the moment.
>
> I have a implementation of 'Member' [4][5] but they are
> unsatisfactory. [4] involves runtime errors and breaking of lens laws.
> [5] shows that we can "create a field" only if we treat Dynamic
> explicitly out of lens context.
>
> I don't know how 'Member' will fit in the lens hierarchy
> http://i.imgur.com/FgfVW.png or even if it fits at all.
> Member is weaker than Getter and probably is a Fold since you have
> only partial getting (^?) for Member. Member is weaker than Prism
> since you cannot construct a whole Object from just one Member. On the
> other hand, setter side of Member requires (s -> b -> t) interface of
> a Lens, because the setter need to update the whole Object when the
> field under concern is missing.
>
> That said, let me put this question in another way;
>
> " We can construct a (Simple Lens s a) from (getter :: s -> a) and
> (setter :: s -> a -> s), and we can construct a (Getter s a) from
> (getter :: s -> a).
>   Then why we cannot construct a (Simple Setter s a) from (setter :: s
> -> a -> s) ? "
>
> Does Member deserve a new node in The Lens Hierarchy tree? Or can we
> implement it by combinations of existing prisms, folds etc? Or does it
> fall outside of the lens framework?
>
> I appreciate any comments.
> Takayuki
>
> [1] https://github.com/nushio3/practice/tree/master/duck
> [2] http://d.hatena.ne.jp/nushio/20121226#p2 (Japanese)
> [3] http://hackage.haskell.org/packages/archive/dynamic-object/0.1.0.1/doc/html/Data-Object-Dynamic-Types.html
> [4] https://github.com/nushio3/practice/blob/master/lens/newfield.hs
> [5] https://github.com/nushio3/practice/blob/master/lens/object-4.hs
>
>
> --
> Takayuki MURANUSHI
> The Hakubi Center for Advanced Research, Kyoto University
> http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html



-- 
Takayuki MURANUSHI
The Hakubi Center for Advanced Research, Kyoto University
http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html