4 ms·

Type classes and generic derivation

42 points by lelf 11y ago

acjohnson55 11y ago
This is a really awesome post for demystifying typeclasses -- which are incredibly useful -- as well as some of the really basic stuff you can do with Shapeless-style type-level programming. The play-json library is how I first started messing with this stuff, but the circe library the author wrote seems to take this to another level. The latter not only seems conceptually and functionally simpler, but is more powerful, given its direct support for sealed trait hierarchies, a core feature of Scala. And it seems to do so in a pretty intuitive way, representing a subtype as a JSON object with the subtype name as the key and a subobject as the value set, as opposed to synthesizing a `type` field of some sort to disambiguate the cases. My biggest complaint is with some of Scala's ergonomics for working on the type level. I'd argue that built-in tuples should simply support the HList operations natively, which seems like it could save on a whole lot of complex looking type-level code. I'm right with the author until `hconsParser` and especially `caseClassParser` started bending my brain. But I guess the good news is that those only need to be written once.
malcolmgreaves 11y ago
Wow! This is an excellent post that really showcases the elegance and expressiveness of a functional programming approach. I personally stumbled upon type classes a few months ago. The pattern upended my style of thinking; definitely greatful for that! I really love how this concept keeps a clean separation between data and functionality. It lends itself to programs that have clear semantics. And makes extending them quite straightforward. Overall great post Travis!
AdieuToLogic 11y ago
Travis Brown has written up some great articles on advanced Scala topics and this one does not disappoint. Well done and worth the read. Some may find the Shapeless feature overview[1] a good read as well. And for people interested in history, I found Mr. Sabin's announcement of Shapeless[2] to be both interesting and valuable for its links related to higher-kinded programming topics. I also found "learning Scalaz"[3] a very good read, with the page discussing Isomorphisms[4] relevant to Mr. Brown's article. 1 - https://github.com/milessabin/shapeless/wiki/Feature-overview%3A-shapeless-2.0.0 https://github.com/milessabin/shapeless/wiki/Feature-overvie... 2 - http://milessabin.com/blog/2011/12/19/shapeless-preview/ http://milessabin.com/blog/2011/12/19/shapeless-preview/ 3 - http://eed3si9n.com/learning-scalaz/ http://eed3si9n.com/learning-scalaz/ 4 - http://eed3si9n.com/learning-scalaz/Isomorphisms.html http://eed3si9n.com/learning-scalaz/Isomorphisms.html

42 points by lelf 11y ago

This is a really awesome post for demystifying typeclasses -- which are incredibly useful -- as well as some of the really basic stuff you can do with Shapeless-style type-level programming. The play-json library is how I first started messing with this stuff, but the circe library the author wrote seems to take this to another level. The latter not only seems conceptually and functionally simpler, but is more powerful, given its direct support for sealed trait hierarchies, a core feature of Scala. And it seems to do so in a pretty intuitive way, representing a subtype as a JSON object with the subtype name as the key and a subobject as the value set, as opposed to synthesizing a `type` field of some sort to disambiguate the cases. My biggest complaint is with some of Scala's ergonomics for working on the type level. I'd argue that built-in tuples should simply support the HList operations natively, which seems like it could save on a whole lot of complex looking type-level code. I'm right with the author until `hconsParser` and especially `caseClassParser` started bending my brain. But I guess the good news is that those only need to be written once.

malcolmgreaves 11y ago

Wow! This is an excellent post that really showcases the elegance and expressiveness of a functional programming approach. I personally stumbled upon type classes a few months ago. The pattern upended my style of thinking; definitely greatful for that! I really love how this concept keeps a clean separation between data and functionality. It lends itself to programs that have clear semantics. And makes extending them quite straightforward. Overall great post Travis!

AdieuToLogic 11y ago

Travis Brown has written up some great articles on advanced Scala topics and this one does not disappoint. Well done and worth the read. Some may find the Shapeless feature overview[1] a good read as well. And for people interested in history, I found Mr. Sabin's announcement of Shapeless[2] to be both interesting and valuable for its links related to higher-kinded programming topics. I also found "learning Scalaz"[3] a very good read, with the page discussing Isomorphisms[4] relevant to Mr. Brown's article. 1 - https://github.com/milessabin/shapeless/wiki/Feature-overview%3A-shapeless-2.0.0 https://github.com/milessabin/shapeless/wiki/Feature-overvie... 2 - http://milessabin.com/blog/2011/12/19/shapeless-preview/ http://milessabin.com/blog/2011/12/19/shapeless-preview/ 3 - http://eed3si9n.com/learning-scalaz/ http://eed3si9n.com/learning-scalaz/ 4 - http://eed3si9n.com/learning-scalaz/Isomorphisms.html http://eed3si9n.com/learning-scalaz/Isomorphisms.html