shapeless – exploring generic programming in Scala Miles Sabin miles.sabin@underscoreconsulting.com http://underscoreconsulting.com/ http://uk.linkedin.com/in/milessabin http://twitter.com/milessabin

Outline ● History ● What is an HList? ● Polymorphic function values ● Type-level functions ● Type- and value-level recursion ● Putting the pieces together ● Find out more ...

History ● Scrap your boilerplate with class in Scala ● Polymorphic function values ● Functional dependencies ● shapeless ● Main goal – explore abstractions over shapes richer than can be captured by traditional parametric polymorphism (even with higher-kinded types) ● Initial objective – an HList which supports map

What is an HList? ● HLists combine the characteristics of both lists and tuples ● Tuples are fixed length sequences of elements of distinct types ● Lists are variable length sequences of elements all of the same type ● HLists are variable length sequences of elements of distinct types ● Code ...

Polymorphic function values ● The first piece of the picture If we're going to map over an HList we need “functions” which can be applied to all of its element types ● Natural transformations? F[T] => G[T] Not general enough ... only gets us to KLists ● SYB gives us the clue Represent polymorphic function values via type classes and instances (taking in singleton and dependent types on the way) ● Code ...

Type-level functions ● Consider the last operation on an HList ● How do we specify its signature? Int :: String :: Boolean :: HNil => Boolean ● We need some way of encoding the relationship between the type of the whole HList and the type of its last element ● Implicits and dependent types to the rescue ● Code ...

Type- and value-level recursion ● What we just saw is an instance of a general pattern ● Type-level recursion “computes” the result type ● Value-level recursion computes the result value ● We do the two in tandem ● Let's look at another slightly more complex example – reverse ● Code ...

Putting the pieces together ● Now we're ready to map that HShit! ● This time our type-level recursion will ● Compute the result type – as before ● Select the appropriate type-specific case of our polymorphic function for each element as we go ● Code ...

Find out more ... ● Fork it! http://github.com/milessabin/shapeless ● Mailing list http://groups.google.com/group/shapeless-dev ● Articles http://chuusai.com/blog

shapeless – exploring generic programming in Scala Miles Sabin miles.sabin@underscoreconsulting.com http://underscoreconsulting.com/ http://uk.linkedin.com/in/milessabin http://twitter.com/milessabin