Hi Kevin! If List conforms to :* -> * then what would be the term for a HOList(High wizardry in the Land of Scala)?
regards Andreas On Sep 3, 12:56 am, Kevin Wright <[email protected]> wrote: > If a thing can be done, then it WILL be done, there's always going to be > somebody out there perverse enough to try it > Almost any tool is open to abuse. The more flexible it is, the more > abusable it becomes: > > http://thereifixedit.failblog.org/ > > C++ templates for example, while a long time favourite of obfuscation > contests, have also resulted in some amazing > work:http://www.boost.org/doc/libs/ > > You'll not be surprised to hear that Scala's type system is no different in > this regard. > > Broadly speaking, there are two important concepts really being explored by > more experienced Scala users; Google on either of these terms with scala, > and you'll come up with a wealth of links: > > "type classes" :http://en.wikipedia.org/wiki/Type_class > <http://en.wikipedia.org/wiki/Type_class>and "kinds" > :http://en.wikipedia.org/wiki/Kind_(type_theory) > > One example of a type class is scala.Numeric, which allows you to define an > algorithm in such a way that it'll operate over ANY type capable of a few > simple operations (plus, minus, divide, etc.) and do so in a completely > type-safe manner, without need of duck typing, etc. > > This is an EXTREMELY powerful abstraction, once you consider that concepts > such as matrices, complex numbers, waveforms, linear equations, etc. can all > be added/subtracted/whatever and so can be considered Numeric. > > Kinds are one level of abstraction up from types. For example, List can be > seen as a Kind * -> * i.e. it takes a type (e.g. Int) and yields a type > (e.g. List[Int]). Map is * -> * -> * (takes two types, and yields one). In > terminology, you'll often see List described as a type constructor, whereas > List[Int] is a Type. List is absolutely not a raw type, Scala doesn't have > such things! > > Scala allows you to generically define an algorithm so that it'll operate > over a particular Kind (not just a specific type). > scalaz, for example, defines a number of operations that can operate over > anything of kind * -> > *http://scalaz.googlecode.com/svn/continuous/latest/browse.sxr/scalaz/... > (disclaimer: This is bleeding edge, brain melting, academic stuff of the > sort that scares people off, it is absolutely NOT day-to-day Scala so please > don't let it put you off! It does however show how far things can be pushed > if you are so minded, not unlike C++ templates then) > > And a few more interesting links, hopefully a lot more approachable than > scalaz :) > > Quick interview with Bill Venners and Martin > Odersky:http://www.artima.com/scalazine/articles/scalas_type_system.html > > Collection of articles on metaprogramming with > types:http://apocalisp.wordpress.com/2010/06/08/type-level-programming-in-s... > > presentation on type > classes:http://lampwww.epfl.ch/~odersky/talks/wg2.8-boston06.pdf > > Generics of a Higher > Kind:http://people.cs.kuleuven.be/~adriaan.moors/files/genericshk/tcpoly.pdf > > video and slides on "High wizardry in the Land of Scala", covering type > classes and Kinds::http://vimeo.com/13518456 > > SO question illustrating a practical application of the Numeric type > class:http://stackoverflow.com/questions/1252915/scala-how-to-define-generi... > > 2010/9/2 Cédric Beust ♔ <[email protected]> > > > > > > > On Thu, Sep 2, 2010 at 2:20 PM, Graham Allan > > <[email protected]>wrote: > > >> One thing that the page doesn't include (AFAICT) is a discussion of what > >> you > >> have referred to elsewhere on this mailing list as a 'Turing-complete type > >> system'. > > > It's the kind of feature that enables hacks such as this > > one<http://en.wikibooks.org/wiki/C%2B%2B_Programming/Templates/Template_M...>. > > If you don't feel like reading it, somebody managed to create C++ templates > > which, when fed to the compiler, will produce error messages that list prime > > numbers. > > > It's awesomely clever but why you would put this as a requirement to > > choosing a language is beyond me. If you don't think that this makes a > > language complex, you are probably hanging out with people who love Haskell > > and who can tell you the difference between a catamorphism and an > > anamorphism in their sleep. > > > -- > > Cédric > > -- > Kevin Wright > > mail / gtalk / msn : [email protected] > pulse / skype: kev.lee.wright > twitter: @thecoda -- You received this message because you are subscribed to the Google Groups "The Java Posse" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/javaposse?hl=en.
