shapeless - exploring generic programming in Scala

Transcript

1. 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
   

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

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3. History
   ●
   Scrap your boilerplate with class in Scala
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   Polymorphic function values
   ●
   Functional dependencies
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   shapeless
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   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
   

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

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5. 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 ...
   

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6. Type-level functions
   ●
   Consider the last operation on an HList
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   How do we specify its signature?
   Int :: String :: Boolean :: HNil => Boolean
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   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 ...
   

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

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

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

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10. 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
    

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