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A completely re-designed collections API was released for Scala 2.8 in 2010. Among it’s innovative features were highly polymorphic methods (e.g. map) whose static types varied in a rich, context-dependent fashion. The technique employed was based upon multi-parameter typeclasses, which interact with the type-inference process. The initial reception was uncertain: what Odersky described as “a new kind of polymorphism”, some Scala users – disturbed by the increased API complexity – described instead as “the longest suicide note in history”. It took another year for Miles Sabin to note the technique had precedent in Haskell, where it was called Functional Dependencies. This talk will explain the technique, and show how it can be useful in applications beyond the Collections API.
Slides and code are available here: https://github.com/benhutchison/TypeclassDrivenPolymorphism
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