Let the Scala compiler work for with you Adelbert Chang 

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Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help ▶ Scala examples: 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help ▶ Scala examples: ▶ Implicits and type classes - proof search 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help ▶ Scala examples: ▶ Implicits and type classes - proof search ▶ Path-dependent types - computing types 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help ▶ Scala examples: ▶ Implicits and type classes - proof search ▶ Path-dependent types - computing types ▶ Literal and singleton types 

Type systems: communicating with the compiler ▶ The type system is our way of talking to the compiler ▶ The more expressive the type system, the more we can say to the compiler ▶ The more the compiler knows, the more it can help ▶ Scala examples: ▶ Implicits and type classes - proof search ▶ Path-dependent types - computing types ▶ Literal and singleton types ▶ Refinement types and (G)ADTs - modeling domains 

Live coding! 

Example: Compiler Errors for Humans1 1http://elm-lang.org/blog/compiler-errors-for-humans 

Example: Compiler Errors for Humans1 Figure 1: https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-come.html 1http://elm-lang.org/blog/compiler-errors-for-humans 

Example: type holes2 2https://phabricator.haskell.org/D3361 

Example: type holes2 ps :: String -> IO () ps = putStrLn ps2 :: a -> IO () ps2 ignored = putStrLn "hello, world" main :: IO () main = _ "hello, world" 2https://phabricator.haskell.org/D3361 

• Found hole: _ :: [Char] -> IO () In the expression: do _ "hello, world" Valid substitutions include ps :: String -> IO () putStrLn :: String -> IO () putStr :: String -> IO () 

Other examples ▶ Friendly compile errors (Elm, Rust, Dotty) 

Other examples ▶ Friendly compile errors (Elm, Rust, Dotty) ▶ Type holes (Haskell, Idris): ask the compiler what we need and what we have 

Other examples ▶ Friendly compile errors (Elm, Rust, Dotty) ▶ Type holes (Haskell, Idris): ask the compiler what we need and what we have ▶ Affine and linear types (Rust, ATS): track ownership and lifetime of variables 

Other examples ▶ Friendly compile errors (Elm, Rust, Dotty) ▶ Type holes (Haskell, Idris): ask the compiler what we need and what we have ▶ Affine and linear types (Rust, ATS): track ownership and lifetime of variables ▶ Refinement types (Liquid Haskell): refine existing types with logical predicates 

Other examples ▶ Friendly compile errors (Elm, Rust, Dotty) ▶ Type holes (Haskell, Idris): ask the compiler what we need and what we have ▶ Affine and linear types (Rust, ATS): track ownership and lifetime of variables ▶ Refinement types (Liquid Haskell): refine existing types with logical predicates ▶ Dependent types (Idris, Agda, Coq): types are values 

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