![Towards a solution
Let’s massage the definition of viewRankings a bit:
What is its type?
So far: Viewers => List[Paper]
From now on: implicit Viewers => List[Paper]](https://image.slidesharecdn.com/curryon-170619122526/85/What-To-Leave-Implicit-26-320.jpg)
What To Leave Implicit
- 1. What to Leave Implicit Martin Odersky Curry On and PLDI Barcelona, June 2017
- 2. Con-textual what comes with the text, but is not in the text
- 3. Context is all around us • the current configuration • the current scope • the meaning of “<” on this type • the user on behalf of which the operation is performed • the security level in effect • …
- 4. • globals rigid if immutable, unsafe if mutable • monkey patching • dependency injection at runtime (Spring, Guice) or with macros (MacWire) • cake pattern close coupling + recursion Traditional ways to express context
- 5. “Parameterize all the things” The Functional Way
- 6. - no side effects - type safe - fine-grained control Functional is Good
- 7. - sea of parameters - most of which hardly ever change - repetitive, boring, prone to mistakes But sometimes it’s too much of a good thing …
- 8. If passing a lot of parameters gets tedious, leave some of them implicit. A more direct approach
- 9. Implicits Types ObjectsFunctions Essence of Scala Synthesis If there’s one feature that makes Scala “Scala”, I would pick implicits.
- 10. • takes you through some common uses of implicits, and some interesting new ones, • goes through a set of language changes that will make implicits even more powerful and safer to use. This Talk t
- 11. • If you do not give an argument to an implicit parameter, one will be provided for you. • Eligible are all implicit values that are visible at the point of call. • If there are more than one eligible candidate, the most specific one is chosen. • If there’s no unique most specific candidate, an ambiguity error Is reported. Ground Rules
- 12. • are a cousin of implicit parameters. • If the type A of an expression does not match the expected type B … … the compiler tries to find an implicit conversion method from A to B. • Same rules as for implicit parameters apply. Implicit Conversions
- 13. • … implicit conversions came first in Scala. • Original motivation: Provide a way for a class to implement new interfaces. • Implicit conversions were at first adopted enthusiastically, but nowadays are mostly discouraged. • I will ignore them in the rest of this talk. Historically …
- 14. Implicit parameters can • prove theorems • establish context • set configurations • inject dependencies • model capabilities • implement type classes Implicit Parameters - Use Cases
- 15. Curry Howard isomorphism: Types = Theorems Programs = Proofs C.f. Kennedy & Russo: “Generalized Type Constraints”, OOPSLA 2004 Prove Theorems
- 16. Establish Context Example: conference management system. Reviewers should only see (directly or indirectly) the scores of papers where they have no conflict with an author.
- 17. Establish Context Example: conference management system. Context is usually stable, can change at specific points.
- 18. Configuration & Dependency Management are special cases of context passing. see also: Dick Wall: The parfait pattern
- 21. Some changes to implicits foreseen for Scala 3: 1. Stricter distinction between parameters and conversions 2. Implicit by-name parameters 3. Multiple implicit parameter lists 4. Implicit function types New Developments
- 22. Multiple Implicit Parameter Lists Problem: Implicit parameters are currently a bit irregular compared to normal parameters: • there can be only one implicit parameter section • and it must come last. This leads to some awkward workarounds (c.f. Aux pattern). Related problem: It’s sometimes confusing when a parameter is implicit or explicit.
- 23. Proposal: • Allow multiple implicit parameter lists • Implicit and explicit parameter lists can be mixed freely. • Explicit application of an implicit parameter must be marked with a new “magic” method, explicitly. Multiple Implicit Parameter Lists
- 24. Implicit Function Types Let’s look at the conference management system again
- 25. Can we do better? Having to write a couple of times does not look so bad. But in dotc there are > 2600 occurrences of the parameter Would it not be nice to get rid of them?
- 26. Towards a solution Let’s massage the definition of viewRankings a bit: What is its type? So far: Viewers => List[Paper] From now on: implicit Viewers => List[Paper]
- 27. Two Rules for Typing 1. Implicit functions get implicit arguments just like implicit methods. Given: f expands to f(a). 2. Implicit functions get created on demand. If the expected type of b is implicit A => B, then b expands to implicit (_: A) => b
- 29. Efficiency Implicit function result types can be optimized Instead of creating a closure like this: we can simply create a curried function like this: This brings the cost of implicit functions down to simple implicit parameters.
- 30. • The reader monad is a somewhat popular method to pass context. • Essentially, it wraps the implicit reading in a monad. • One advantage: The reading is abstracted in a type. • But I believe this is shooting sparrows with cannons. • Monads are about sequencing, they have have nothing to do with passing context. The Reader Monad
- 31. • allow the same conciseness as the reader monad, • don’t force you into monadic style with explicit sequencing, • are fully composable, • are more than 7x faster than the reader monad. Implicit Function Types
- 32. Neat way to define structure-building DSLs, like this: Natively supported in Groovy and in Kotlin via “receiver functions”. An Encore: The Builder Pattern
- 34. A Formalization Bidirectional typing rules: Γ ⊢ t :▷ T (Synthesis) Γ ⊢ t ◁: T (Checking)
- 35. SI Typing Rules: Functions
- 36. SI Typing Rules: Variables
- 37. SI Typing Rules: Let
- 38. SI Typing Rules: Everything
- 39. Translation to System F
- 40. • Implicit parameters in Haskell (Lewis et al, 2000) • Modular implicits in OCaml (White, Bour, Yallop, 2015) • Agda’s implicit instances (Devriese and Piessens, 2011) - very similar to implicit function types. • Implicit Calculus by (Olivera, Schrjvers, Wadler et al., 2012, 2017) - a bit further removed from Scala than SI. - Foundations of Implicit Function Types (Odersky, Biboudis, Liu, Blanvillain) EPFL Report 229203, 2017 References
- 41. Dotty: Dmitry Petrashko Nicolas Stucki Guillaume Martres Felix Mulder Ondrej Lhotak Aggelos Biboudis Liu Fengyun Vera Salvis Olivier Blanvillain Enno Runne Sebastien Douraene … and many others DOT: Nada Amin Tiark Rompf Sandro Stucki Samuel Grütter scalac at Adriaan Moors Seth Tisue Lightbend: Jason Zaugg Stefan Zeiger Lukas Rytz Scala Heather Miller Julien Richard-Foy Center: Jorge Cantero Olafur Geirsson Guillaume Massé Martin Duheim Travis Lee Credits Thank You