Lenses for nested sum types e.g. Either.
On 03/10/2013 6:08 PM, "Simon Peyton-Jones"
(I sent this to ‘libraries’ but Kim-Ee suggested adding Café, where so many smart people hang out.)****
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Friends****
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Some of you will know that I’ve promised to give a talk about Edward’s lens library http://hackage.haskell.org/package/lens at the Haskell Exchange http://skillsmatter.com/event/scala/haskell-exchange in London next Wednesday (9th). I did this to give everyone (including me) a break from GHC hackery, and also to force me to learn about this lens voodoo that everyone is twittering about. Edward generously gave me quite a bit of one-to-one attention last week (my hair is still standing on end), but this message is to ask your help too.****
*Specifically, I’d like to give some compelling use-cases*. If you are using the lens library yourself, could you spare a few minutes to tell me how you are using it? I expect to cover Lens and Traversal but not Prism. ****
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The use-case everyone starts with is nested records, but I’d like to go beyond that. The next levels seem to be:****
**· **Lenses as views of data that isn’t “really there” e.g. regarding a record with rectangular coordinates as having polar coordinates too.****
**· **Lenses and Traversals that focus on elements of finite maps (Control.Lens.At)****
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What else? I’m sure you are using them in all sorts of cool ways that I would never think of, and I’d love to know.****
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Please don’t tell me anything secret! To give everyone the benefit I may just concatenate all the replies and send to you all, so please say if you don’t want me to do that with yours. ****
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And don’t burn too many cycles on this...I don’t want to waste your time, and I can always get back to you if I can’t understand what you say. Sooner is better than later...Weds is coming.****
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Simon “Edward’s prophet” PJ****
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