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Operation
Haskell + Scala
Towards Hybrid Functional Development
Haskell + Scala ハイブリッド開発大作戦
Cheshire Cat (@y_taka_23)
ScalaMatsuri 2018 (2018/03/17)
#ScalaMatsuri
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#ScalaMatsuri
@y_taka_23
ご意見ご感想は Twitter へ
#ScalaMatsuri
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Haskell
みなさんご存知 Haskell
http://haskell.org
#ScalaMatsuri
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Eta
でも今日の題材は Eta
https://eta-lang.org
#ScalaMatsuri
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What’s Eta Lang?
● Haskell on JVM
○ OSS by Typelead Inc. in India
○ Highly compatible with Haskell
● Generates Java bytecode
○ Smooth interoperation with JVM languages
● Forked from GHC
○ The de-facto standard Haskell compiler
Eta は JVM 上で動く Haskell
GHC からのフォークで、バイトコードを生成
#ScalaMatsuri
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Contents
● Purity in Haskell
○ Haskell, what a procedural language
○ What is the biggest difference from Scala?
● Java/Scala interoperation
○ Java Monads
○ Type-level programming
● Example of hybrid-project building
純粋性、Java/Scala との相互呼び出し
ハイブリッドプロジェクトの例
#ScalaMatsuri
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1.
Purity in Haskell
Haskell の純粋性
Quarantine side-effects
#ScalaMatsuri
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Maybe You Guys Think...
● Haskell is too complicated
○ Functional programing is Greek to me...
○ That’s a language for mathematicians…
○ Category theory...
● Haskell is poor at IO
○ Even “Hello, world” requires understanding of monads
○ Never be a production-ready language
Haskell は数学的で難解という先入観
#ScalaMatsuri
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Naive FizzBuzz in Scala
for (i <- 1 to 100) {
if (i % 3 == 0 && i % 5 == 0) {
println("FizzBuzz")
} else if (i % 3 == 0) {
println("Fizz")
} else if (i % 5 == 0) {
println("Buzz")
} else {
println(i)
}
}
Scala による単純な FizzBuzz
#ScalaMatsuri
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Also Naive FuzzBuzz in Haskell
main :: IO ()
main = do
forM_ [1..100] $ \i -> do
if i `mod` 3 == 0 && i `mod` 5 == 0 then
putStrLn "FizzBuzz"
else if i `mod` 3 == 0 then
putStrLn "Fizz"
else if i `mod` 5 == 0 then
putStrLn "Buzz"
else
putStrLn $ show i
Haskell による単純な FizzBuzz
#ScalaMatsuri
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Also Naive FuzzBuzz in Haskell
main :: IO ()
main = do
forM_ [1..100] $ \i -> do
if i `mod` 3 == 0 && i `mod` 5 == 0 then
putStrLn "FizzBuzz"
else if i `mod` 3 == 0 then
putStrLn "Fizz"
else if i `mod` 5 == 0 then
putStrLn "Buzz"
else
putStrLn $ show i
Haskell による単純な FizzBuzz
for-loop
#ScalaMatsuri
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Also Naive FuzzBuzz in Haskell
main :: IO ()
main = do
forM_ [1..100] $ \i -> do
if i `mod` 3 == 0 && i `mod` 5 == 0 then
putStrLn "FizzBuzz"
else if i `mod` 3 == 0 then
putStrLn "Fizz"
else if i `mod` 5 == 0 then
putStrLn "Buzz"
else
putStrLn $ show i
Haskell による単純な FizzBuzz
if-conditional
#ScalaMatsuri
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Also Naive FuzzBuzz in Haskell
main :: IO ()
main = do
forM_ [1..100] $ \i -> do
if i `mod` 3 == 0 && i `mod` 5 == 0 then
putStrLn "FizzBuzz"
else if i `mod` 3 == 0 then
putStrLn "Fizz"
else if i `mod` 5 == 0 then
putStrLn "Buzz"
else
putStrLn $ show i
Haskell による単純な FizzBuzz
stdout
#ScalaMatsuri
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Haskell is Not Rocket Science
● Procedural programming
○ You don’t need to be too “functional”
○ Remember Scala as “better Java”
○ Naive sequantials / conditionals / loops
● Mathematical background is not mandatory
○ HelloWorld / FizzBuzz In the usual way
● What’s difference of Haskell from Scala?
Haskell は手続き的なコードも書ける
では Scala との違いは何か？
#ScalaMatsuri
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Scala における純粋・非純粋なメソッド
def pureMessage(name: String): String = {
"Hello, " ++ name
}
def inpureMessage(name: String): String = {
val another = io.StdIn.readLine()
"Hello, " ++ name ++ ", " ++ another
}
#ScalaMatsuri
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Scala における純粋・非純粋なメソッド
def pureMessage(name: String): String = {
"Hello, " ++ name
}
def inpureMessage(name: String): String = {
val another = io.StdIn.readLine()
"Hello, " ++ name ++ ", " ++ another
}
side-effect
#ScalaMatsuri
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Scala における純粋・非純粋なメソッド
def pureMessage(name: String): String = {
"Hello, " ++ name
}
def inpureMessage(name: String): String = {
val another = io.StdIn.readLine()
"Hello, " ++ name ++ ", " ++ another
}
#ScalaMatsuri
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Haskell における純粋・非純粋な関数
pureMessage :: String -> String
pureMessage name =
"Hello, " ++ name
inpureMessage :: String -> String
inpureMessage name = do
another <- getLine
return $ "Hello, " ++ name ++ ", " ++ another
#ScalaMatsuri
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Haskell における純粋・非純粋な関数
pureMessage :: String -> String
pureMessage name =
"Hello, " ++ name
inpureMessage :: String -> String
inpureMessage name = do
another <- getLine
return $ "Hello, " ++ name ++ ", " ++ another
type error
#ScalaMatsuri
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Haskell における純粋・非純粋な関数
pureMessage :: String -> String
pureMessage name =
"Hello, " ++ name
inpureMessage :: String -> IO String
inpureMessage name = do
another <- getLine
return $ "Hello, " ++ name ++ ", " ++ another
compilable
#ScalaMatsuri
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Haskell における純粋・非純粋な関数
pureMessage :: String -> String
pureMessage name =
"Hello, " ++ name
inpureMessage :: String -> IO String
inpureMessage name = do
another <- getLine
return $ "Hello, " ++ name ++ ", " ++ another
#ScalaMatsuri
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IO as a Type in Haskell
● Side effects should be separated
○ Regardless of which programming languages
○ Not predictable nor testable
○ No built-in methodology for dealing IO in Scala
● In Haskell, it appears as a type
○ Compiler can check it at compilation time
○ No careless side effects
Haskell では IO が型に現れるため
コンパイラがチェック可能
#ScalaMatsuri
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Recap of Section. 1
● Haskell in the naive ways
○ Procedural programming if you want
○ Without (explicit) math objects, like monads
● Side effects appear as the IO Type
○ Compiler can check the “real world dependency”
○ Quarantine “impure” parts from “pure” parts
Haskell は手続き的な記述も可能だが
副作用が型として見える点で一線を画す
#ScalaMatsuri
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2.
Interoperation
Java/Scala との相互呼び出し
Levarate by JVM ecosystem
#ScalaMatsuri
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Advantages of JVM Languages
● Platform independent
○ 3 billion devices run JVM
● Mature ecosystem
○ Reuse your existing in-house/3rd party libraries
● Gradual/partial introduction
○ Small start, without change of infrastructure
○ Huge market for Java developers
既存のエコシステムに乗ることで
移行コストを低くできるのは JVM 言語の利点
#ScalaMatsuri
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各言語間の相互呼び出し
#ScalaMatsuri
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Java/Scala から Eta
#ScalaMatsuri
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Exporting Eta Functions
module Util where
import Java
capitalize :: JString -> JString
capitalize = ...
foreign export java "@eta.example.Util.capitalize"
capitalize :: JString -> JString
Eta の関数のエクスポート
#ScalaMatsuri
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Exporting Eta Functions
module Util where
import Java
capitalize :: JString -> JString
capitalize = ...
foreign export java "@eta.example.Util.capitalize"
capitalize :: JString -> JString
Eta の関数のエクスポート
@FQDN.functionName
#ScalaMatsuri
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Importing as a Static Method
import eta.example.Util
object Main {
def main(args: Array[String]): Unit = {
val str = "hello, eta interop!"
println(Util.capitalize(str))
}
}
Java/Scala 側からは静的メソッドに見える
#ScalaMatsuri
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Importing as a Static Method
import eta.example.Util
object Main {
def main(args: Array[String]): Unit = {
val str = "hello, eta interop!"
println(Util.capitalize(str))
}
}
Java/Scala 側からは静的メソッドに見える
singleton method
#ScalaMatsuri
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Eta から Java
#ScalaMatsuri
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Two Big Hurdles in Java Interop
● State handling
○ In OOP, objects have their internal states
○ Java methods are potentially polluted by IO
○ It looks conflicting with the purity in Haskell
● Class inheritance
○ Haskell doesn’t have subtyping
○ Many Java stuffs depend on inheritance
Java 連携で問題になるのは状態の扱いと継承
#ScalaMatsuri
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How can we deal mutable objects?
可変オブジェクトの扱いは？
#ScalaMatsuri
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flatMap
Scala の flatMap
#ScalaMatsuri
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Toy Model of of “States”
● Manipulate a stack
○ The Simplest example of internal states
● Simplify the model
○ Contents are integers
○ Pop an integer from the top of stack
○ Push an integer x on the top of stack
● Emulate the stack as pure functions
単純化した「状態」のモデル
#ScalaMatsuri
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Requirements for Stack Manipulation
● Stack is immutable per se
○ Need to return the “modified” stack as a result
● Concatenate sequential actions
○ “Doing act1, and then act2” should be an action
● Action depends on actions in the past
○ “Pop x and then pop y, and then push (x + y)”
欲しい Stack 操作の条件
イミュータブル、逐次処理、以前の結果の利用
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
pop :: Stack -> (Int, Stack)
pop (x : xs) = (x, xs)
push :: Int -> Stack -> ((), Stack)
push x = ((), x : xs)
変更されたスタックも一緒に返す
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
pop :: Stack -> (Int, Stack)
pop (x : xs) = (x, xs)
push :: Int -> Stack -> ((), Stack)
push x = ((), x : xs)
変更されたスタックも一緒に返す
type synonym
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
pop :: Stack -> (Int, Stack)
pop (x : xs) = (x, xs)
push :: Int -> Stack -> ((), Stack)
push x = ((), x : xs)
変更されたスタックも一緒に返す
(result, modified stack)
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
pop :: Stack -> (Int, Stack)
pop (x : xs) = (x, xs)
push :: Int -> Stack -> ((), Stack)
push x = ((), x : xs)
変更されたスタックも一緒に返す
result is Unit
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
pop :: Stack -> (Int, Stack)
pop (x : xs) = (x, xs)
push :: Int -> Stack -> ((), Stack)
push x = ((), x : xs)
変更されたスタックも一緒に返す
#ScalaMatsuri
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Immutable Stack
type Stack = [Int]
type Action a = Stack -> (a, Stack)
pop :: Action Int
pop (x : xs) = (x, xs)
push :: Int -> Action ()
push x = ((), x : xs)
変更されたスタックも一緒に返す
#ScalaMatsuri
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Sequential Actions
andThen :: Action a -> Action b -> Action b
andThen act1 act2 = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = act2 stack1
in (x2, stack2)
myAct :: Action ()
myAct = push 1 `andThen` push 2 `andThen` push 3
操作を連結した結果も操作
#ScalaMatsuri
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Sequential Actions
andThen :: Action a -> Action b -> Action b
andThen act1 act2 = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = act2 stack1
in (x2, stack2)
myAct :: Action ()
myAct = push 1 `andThen` push 2 `andThen` push 3
操作を連結した結果も操作
concatenation
#ScalaMatsuri
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Sequential Actions
andThen :: Action a -> Action b -> Action b
andThen act1 act2 = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = act2 stack1
in (x2, stack2)
myAct :: Action ()
myAct = push 1 `andThen` push 2 `andThen` push 3
操作を連結した結果も操作
pass the modified stack
#ScalaMatsuri
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Reuse results of the previous actions
andThen :: Action a -> (a -> Action b)
-> Action b
andThen act1 factory = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = (factory x1) stack1
in (x2, stack2)
myAct :: Action ()
myAct = pop `andThen` (\x ->
pop `andThen` (\y ->
push (x + y)))
以前の結果に依存した操作
#ScalaMatsuri
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Reuse results of the previous actions
andThen :: Action a -> (a -> Action b)
-> Action b
andThen act1 factory = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = (factory x1) stack1
in (x2, stack2)
myAct :: Action ()
myAct = pop `andThen` (\x ->
pop `andThen` (\y ->
push (x + y)))
以前の結果に依存した操作
#ScalaMatsuri
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Reuse results of the previous actions
andThen :: Action a -> (a -> Action b)
-> Action b
andThen act1 factory = \stack ->
let (x1, stack1) = act1 stack
(x2, stack2) = (factory x1) stack1
in (x2, stack2)
myAct :: Action ()
myAct = pop `andThen` (\x ->
pop `andThen` (\y ->
push (x + y)))
以前の結果に依存した操作
depends on x1
#ScalaMatsuri
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andThen :: Action a -> (a -> Action b)
-> Action b
最終的に得られた関数
#ScalaMatsuri
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Option[A]#
flatmap[B](A => Option[B]): Option[B]
Scala の flatMap
#ScalaMatsuri
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Action[A]#
flatmap[B](A => Action[B]): Action[B]
Scala の flatMap
#ScalaMatsuri
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flatMap :: Action[A] -> (A -> Action[B])
-> Action[B]
Scala の flatMap
#ScalaMatsuri
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andThen :: Action a -> (a -> Action b)
-> Action b
最終的に得られた関数
#ScalaMatsuri
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(>>=) :: m a -> (a -> m b) -> m b
Monad 型クラスの bind 関数
#ScalaMatsuri
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Scala’s for-Comprehension
// actual
opt1.flatMap(x => opt2.map(y => x + y))
// sugared
for {
x <- opt1
y <- opt2
} yield {
x + y
}
Scala の for 式
#ScalaMatsuri
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Haskell’s do-Notation
-- actual
myAct = pop >>= (\x -> pop >>= (\y -> push (x + y)))
-- sugared
myAct :: Action ()
myAct = do
x <- pop
y <- pop
push (x + y)
Haskell の do 記法
#ScalaMatsuri
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Haskell’s do-Notation
-- actual
myAct = pop >>= (\x -> pop >>= (\y -> push (x + y)))
-- sugared
myAct :: Action ()
myAct = do
x <- pop
y <- pop
push (x + y)
Haskell の do 記法
#ScalaMatsuri
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Haskell’s do-Notation
-- actual
myAct = pop >>= (\x -> pop >>= (\y -> push (x + y)))
-- sugared
myAct :: Action ()
myAct = do
x <- pop
y <- pop
push (x + y)
Haskell の do 記法
hidden Stack
#ScalaMatsuri
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What’s the Java Monads?
● DSL for the interop
○ Use monads as a framework
● Code block of a type “Java a b”
○ a: Type of the “implicit” method receiver
○ b: Type of the final execution result
○ “Java a” is a monad, depending on a
● IO is a special case of Java monads
Java モナドは Java 呼び出しのための DSL
レシーバと戻り値の組で型が決まる
#ScalaMatsuri
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-- add :: e -> Java (Deque e) Boolean
-- poll :: Java (Deque e) e
myAct1 :: Java (Deque Int) Boolean
myAct1 = add 1
myAct2 :: Java (Deque Int) Bool
myAct2 = do
add 2
add 1
x <- poll
y <- poll
add (x + y)
Deque クラスによる Java モナドの例
#ScalaMatsuri
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-- add :: e -> Java (Deque e) Boolean
-- poll :: Java (Deque e) e
myAct1 :: Java (Deque Int) Boolean
myAct1 = add 1
myAct2 :: Java (Deque Int) Bool
myAct2 = do
add 2
add 1
x <- poll
y <- poll
add (x + y)
Deque クラスによる Java モナドの例
hidden receiver :: Deque Int
#ScalaMatsuri
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Instance Methods
data File = File @java.io.File
deriving Class
foreign import java unsafe
"canExecute"
canExecute :: Java File Bool
インスタンスメソッドのインポート
#ScalaMatsuri
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Instance Methods
data File = File @java.io.File
deriving Class
foreign import java unsafe
"canExecute"
canExecute :: Java File Bool
インスタンスメソッドのインポート
typeclass as a marker
#ScalaMatsuri
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Instance Methods
data File = File @java.io.File
deriving Class
foreign import java unsafe
"canExecute"
canExecute :: Java File Bool
インスタンスメソッドのインポート
receiver :: File
return value :: Bool
#ScalaMatsuri
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Constructors
data File = File @java.io.File
deriving Class
foreign import java unsafe
"@new"
newFile :: String -> Java a File
コンストラクタのインポート
#ScalaMatsuri
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Constructors
data File = File @java.io.File
deriving Class
foreign import java unsafe
"@new"
newFile :: String -> Java a File
コンストラクタのインポート
no receiver
return value :: File
#ScalaMatsuri
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Static Methods
data File = File @java.io.File
deriving Class
foreign import java unsafe
"@static java.io.File.createTempFile"
createTempFile :: String -> String -> Java a File
静的メソッドのインポート
#ScalaMatsuri
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Static Methods
data File = File @java.io.File
deriving Class
foreign import java unsafe
"@static java.io.File.createTempFile"
createTempFile :: String -> String -> Java a File
静的メソッドのインポート
no receiver
return value :: File
#ScalaMatsuri
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Glue of Java Monads
java :: Java c a -> IO a
javaWith :: (Class c) => c -> Java c a -> IO a
<.> :: (Class c) => c -> Java c a -> Java b a
>- :: (Class b) => Java a b -> Java b c -> Java a c
io :: IO c -> Java c a
メソッドレシーバーを入れ替える関数群
#ScalaMatsuri
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The other hurdle: Inheritance
もう一つの問題、継承関係
#ScalaMatsuri
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foreign import java unsafe "@new"
newFile :: String -> Java a File
foreign import java unsafe
toString :: Object -> String
data File = File @java.io.File
deriving Class
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係によるコンパイルエラー
#ScalaMatsuri
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foreign import java unsafe "@new"
newFile :: String -> Java a File
foreign import java unsafe
toString :: Object -> String
data File = File @java.io.File
deriving Class
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係によるコンパイルエラー
#ScalaMatsuri
file :: File
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foreign import java unsafe "@new"
newFile :: String -> Java a File
foreign import java unsafe
toString :: Object -> String
data File = File @java.io.File
deriving Class
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係によるコンパイルエラー
#ScalaMatsuri
type error
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{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
foreign import java unsafe
toString :: (a <: Object) => a -> String
type instance Inherits File = '[Object]
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係を踏まえた関数呼び出し
#ScalaMatsuri
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{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
foreign import java unsafe
toString :: (a <: Object) => a -> String
type instance Inherits File = '[Object]
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係を踏まえた関数呼び出し
#ScalaMatsuri
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GHC Extensions
● GHC specialities
○ Experimental futures, out of the language spec
○ Eta, which is a fork of GHC, can use them
● Enabled by special comments
○ {-# LANGUAGE ExtensionNames #-}
● Unavoidable for modern Haskell
○ Especially abstract frameworks
Eta は GHC のフォークなので
コンパイラ拡張が使用可能
#ScalaMatsuri
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Extensions for Inheritance
● DataKinds
○ Promote normal types to kinds
● TypeFamilies
○ Define type-level functions
● TypeOperators
○ Declare an infix operator for a 2-param typeclass
継承関係を表現するための三つの拡張
#ScalaMatsuri
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Types and Kinds
● Kinds are “types of types”
○ 1, 2, 3, … :: Int
○ Int, String, File, ... :: *
○ Maybe :: * -> *
● DataKinds promote datatypes
○ Types to kinds, data constructors to types
○ Enable to use values in type-level functions
カインドは「型の型」
DataKinds 拡張でデータ型を昇格できる
#ScalaMatsuri
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TypeFamilies
type family Inherits (a :: *) :: [*]
data Defined = Yes | No
type family Extends'
(a :: Class) (b :: Class) :: Defined where
Extends' a a = Yes
Extends' a Object = Yes
Extends' Object a = No
Extends' a b = ExtendsList (Inherits a) b
TypeFamilies 拡張を用いた型レベル計算
#ScalaMatsuri
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TypeFamilies
type family Inherits (a :: *) :: [*]
data Defined = Yes | No
type family Extends'
(a :: Class) (b :: Class) :: Defined where
Extends' a a = Yes
Extends' a Object = Yes
Extends' Object a = No
Extends' a b = ExtendsList (Inherits a) b
TypeFamilies 拡張を用いた型レベル計算
type-level function
#ScalaMatsuri
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TypeFamilies
type family Inherits (a :: *) :: [*]
data Defined = Yes | No
type family Extends'
(a :: Class) (b :: Class) :: Defined where
Extends' a a = Yes
Extends' a Object = Yes
Extends' Object a = No
Extends' a b = ExtendsList (Inherits a) b
TypeFamilies 拡張を用いた型レベル計算
promoted kind
#ScalaMatsuri
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TypeFamilies
type family Inherits (a :: *) :: [*]
data Defined = Yes | No
type family Extends'
(a :: Class) (b :: Class) :: Defined where
Extends' a a = Yes
Extends' a Object = Yes
Extends' Object a = No
Extends' a b = ExtendsList (Inherits a) b
TypeFamilies 拡張を用いた型レベル計算
a extends b?
#ScalaMatsuri
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TypeFamilies
type family ExtendsList
(a :: [*]) (b :: *) :: Defined where
ExtendsList '[] y = No
ExtendsList (x ': xs) y =
Or (Extends' x y) (ExtendsList xs y)
type family Or
(a :: Defined) (b :: Defined) :: Defined where
Or No No = No
Or a b = Yes
TypeFamilies 拡張を用いた型レベル計算
#ScalaMatsuri
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TypeFamilies
type family ExtendsList
(a :: [*]) (b :: *) :: Defined where
ExtendsList '[] y = No
ExtendsList (x ': xs) y =
Or (Extends' x y) (ExtendsList xs y)
type family Or
(a :: Defined) (b :: Defined) :: Defined where
Or No No = No
Or a b = Yes
TypeFamilies 拡張を用いた型レベル計算
(one of a) extends b?
#ScalaMatsuri
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TypeOperators
class (Class a, Class b) => Extends a b where
superCast :: a -> b
unsafeCast :: b -> a
instance (Class a, Class b, Extends' a b ~ Yes)
=> Extends a b where
type (<:) a b = Extends a b
TypeOperators 拡張による型演算子の定義
#ScalaMatsuri
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TypeOperators
class (Class a, Class b) => Extends a b where
superCast :: a -> b
unsafeCast :: b -> a
instance (Class a, Class b, Extends' a b ~ Yes)
=> Extends a b where
type (<:) a b = Extends a b
TypeOperators 拡張による型演算子の定義
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{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
foreign import java unsafe
toString :: (a <: Object) => a -> String
type instance Inherits File = '[Object]
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係を踏まえた関数呼び出し
#ScalaMatsuri
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{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
foreign import java unsafe
toString :: (a <: Object) => a -> String
type instance Inherits File = '[Object]
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係を踏まえた関数呼び出し
TypeOperators
#ScalaMatsuri
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{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
foreign import java unsafe
toString :: (a <: Object) => a -> String
type instance Inherits File = '[Object]
main :: IO ()
main = do
file <- java $ newFile "text.txt"
putStrLn $ toString file
継承関係を踏まえた関数呼び出し
TypeFamilies
#ScalaMatsuri
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Recap of Section. 2
● Exposting Eta functions
○ Straightforward, just as static methods
● Java monads
○ Method receiver can be considered as a monad
● inheritance
○ Type-level definitions by the GHC extensions
Eta からエクスポートはそのまま
インポートは Java モナドと型レベル記述を活用
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3.
HYBRID PROJECT
ハイブリッド・プロジェクトの構成
Why composability matters?
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Eta から Scala
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object Client {
implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()
implicit val execContext = system.dispatcher
val responseFuture: Future[HttpResponse] =
Http().singleRequest(
HttpRequest(uri = "https://akka.io"))
responseFuture.onComplete {
case Success(res) => println(res)
case Failure(_) => sys.error("ERROR!")
}
}
Scala による Akka HTTP Client のサンプル
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apply メソッドの糖衣構文
// sugared
val system = ActorSystem()
// actual
val system = ActorSystem.apply()
data ActorSystem = ...
foreign import java unsafe
"@static akka.actor.ActorSystem.apply"
newActorSystem :: Java a ActorSystem
#ScalaMatsuri
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apply メソッドの糖衣構文
// sugared
val system = ActorSystem()
// actual
val system = ActorSystem.apply()
data ActorSystem = ...
foreign import java unsafe
"@static akka.actor.ActorSystem.apply"
newActorSystem :: Java a ActorSystem
static method
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// sugared
val materializer = ActorMaterializer()
/* actual
*
* ActorMaterializer.apply(
* settings: Option[MaterializerSettings] = None,
* prefix: Option[String] = None
* ) (implicit context: ActorRefFactory)
*/
val materializer =
ActorMaterializer(None, None, system)
実は大量のデフォルト・暗黙引数
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// sugared
val materializer = ActorMaterializer()
/* actual
*
* ActorMaterializer.apply(
* settings: Option[MaterializerSettings] = None,
* prefix: Option[String] = None
* ) (implicit context: ActorRefFactory)
*/
val materializer =
ActorMaterializer(None, None, system)
実は大量のデフォルト・暗黙引数
default / implicit args
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data Option a = Option (@scala.Option a)
data ActorRefFactory = ...
data ActorMaterializerSettings = ...
type instance Inherits ActorSystem =
'[Object, ActorRefFactory]
foreign import java unsafe
"@static akka.stream.ActorMaterializer.apply"
newActorMaterializer
:: (a <: ActorRefFactory)
=> Option ActorMaterializerSettings
-> Option JString -> a
-> Java c ActorMaterializer
明示的なインポートが必要
#ScalaMatsuri
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Annoying
めんどい
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Difficulty in Scala Interop
● Eta interop is “sugar free”
○ Designed for Java interoperation
○ Poor support for commonly-used Scala data types
● Lots of boilerplates required
○ Backfire of Scala’s sophisticated features
○ Code generator is not well-developed
● Better to use Java libraries
Eta から Scala のライブラリを呼ぶと
ボイラプレートが増えがち
#ScalaMatsuri
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Best Practice for Hybrid Projects
● Phase 1: Eta inside Scala
○ Eta is only for pure logic, like data transformation
○ Scala parts invokes Eta
○ Communicate with outside by Scala
● Phase 2: Java Inside Eta
○ Eta is for main parts of the service
○ Eta invokes Java, to communicate others
第一段階：Scala 内に Eta を埋め込む
第二段階：Eta 内に Java を埋め込む
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
Framework
Micro Service
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
Framework
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
Framework
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
Framework
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
Framework
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
REST, gRPC
#ScalaMatsuri
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Gradual Hybriding Strategy
段階的な置き換え戦略
REST, gRPC
#ScalaMatsuri
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Etlas
● Standard Build Tool for Eta
○ Forked from Cabal in Haskell
○ Similar to sbt in Scala
● Dependency manager + task runner
○ Common build definition with Haskell (.cabal)
○ Accesses official Haskell package repos (Hackage)
○ Fetches patches from eta-hackage
Eta 用ビルドツール Etlas
Haskell と共通の仕組みを利用
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Toy Example
● Microservice written in Scala
○ On Scala pipeline, built by sbt
● Transform JSON
○ Consider records like { “name”: “alice”, “age”: 17 }
○ Capitalize “name” fields
○ Add 1 to the “age” fields
簡単なサンプルプロジェクト
#ScalaMatsuri
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Embed Eta into Scala
Scala プロジェクトに Eta を埋め込む
Framework
#ScalaMatsuri
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Directory Structure
+-- project
| +-- plugins.sbt
+-- src
+-- main
+-- scala
| +-- example
| +-- Main.scala
+-- eta
+-- example.cabal
+-- Example
+-- Transform.hs
プロジェクトの構成
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Directory Structure
+-- project
| +-- plugins.sbt
+-- src
+-- main
+-- scala
| +-- example
| +-- Main.scala
+-- eta
+-- example.cabal
+-- Example
+-- Transform.hs
プロジェクトの構成
embedded project
#ScalaMatsuri
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Cabal File for Etlas
library
build-depends: base
, lens
exported-moduels: Example.Transform
default-language: Haskell2010
Etlas 用ビルド定義
#ScalaMatsuri
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Cabal File for Etlas
library
build-depends: base
, lens
exported-moduels: Example.Transform
default-language: Haskell2010
Etlas 用ビルド定義
Haskell library
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module Example.Transform where
fixJson :: JString -> JString
fixJson = toJava . transform . fromJava
where
transform :: Text -> Text
transform json = json
& (key "name" . _String) %~ toUpper
& (key "age" . _Integral) %~ (+1)
foreign export java
"@static eta.example.Transform.fixJson"
fixJson :: JString -> JString
Eta 側：純粋なロジック
#ScalaMatsuri
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module Example.Transform where
fixJson :: JString -> JString
fixJson = toJava . transform . fromJava
where
transform :: Text -> Text
transform json = json
& (key "name" . _String) %~ toUpper
& (key "age" . _Integral) %~ (+1)
foreign export java
"@static eta.example.Transform.fixJson"
fixJson :: JString -> JString
Eta 側：純粋なロジック
#ScalaMatsuri
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package example
import eta.example.Transform
object Main extends App {
val writer = new PrintWriter(new File("out.json"))
Source.fromResource("in.json")
.getLine.foreach { json =>
writer.write(Transform.fixJson(json) ++ "\n")
}
writer.close()
}
Scala 側：外界との inpure なやりとり
#ScalaMatsuri
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package example
import eta.example.Transform
object Main extends App {
val writer = new PrintWriter(new File("out.json"))
Source.fromResource("in.json")
.getLine.foreach { json =>
writer.write(Transform.fixJson(json) ++ "\n")
}
writer.close()
}
Scala 側：外界との inpure なやりとり
singleton method
#ScalaMatsuri
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sbt Plugin for Etlas
+-- project
| +-- plugins.sbt
+-- src
+-- main
+-- scala
| +-- example
| +-- Main.scala
+-- eta
+-- example.cabal
+-- Example
+-- Transform.hs
プラグインの導入
#ScalaMatsuri
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sbt Plugin for Etlas
+-- project
| +-- plugins.sbt
+-- src
+-- main
+-- scala
| +-- example
| +-- Main.scala
+-- eta
+-- example.cabal
+-- Example
+-- Transform.hs
プラグインの導入
addSbtPlugin("com.typelead" % "sbt-eta" % "0.1.0")
#ScalaMatsuri
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Embed Java into Eta
Eta プロジェクトに Java を埋め込む
Framework
#ScalaMatsuri
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Directory Structure
+-- example.cabal
+-- src
| +-- Main.hs
+-- java
+-- example
+-- Util.java
プロジェクトの構成
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Directory Structure
+-- example.cabal
+-- src
| +-- Main.hs
+-- java
+-- example
+-- Util.java
プロジェクトの構成
embedded project
#ScalaMatsuri
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Cabal File for Etlas
executable eta-example
main-is: Main.hs
hs-source-dirs: src
build-depends: base
default-language: Haskell2010
java-sources: java/example/Util.java
, /path/to/Util.class
, /path/to/Util.jar
maven-depends: com.example:awesome:x.y.z
Etlas 用ビルド定義
#ScalaMatsuri
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Cabal File for Etlas
executable eta-example
main-is: Main.hs
hs-source-dirs: src
build-depends: base
default-language: Haskell2010
java-sources: java/example/Util.java
, /path/to/Util.class
, /path/to/Util.jar
maven-depends: com.example:awesome:x.y.z
Etlas 用ビルド定義
Java stuffs
#ScalaMatsuri
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Cabal File for Etlas
executable eta-example
main-is: Main.hs
hs-source-dirs: src
build-depends: base
default-language: Haskell2010
java-sources: java/example/Util.java
, /path/to/Util.class
, /path/to/Util.jar
maven-depends: com.example:awesome:x.y.z
Etlas 用ビルド定義
Maven repository
#ScalaMatsuri
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Recap of Section. 3
● Scala libraries are hard to use from Eta
○ Eta cannot handle Scala’s syntactic sugar
○ In the most cases, Java version is better to import
● Follow the best practice
○ Firstly embed Eta into Scala
○ Then embed Java into Eta
● sbt, Maven repos + Etlas integration
Eta から呼ぶなら Scala ではなく Java
ビルドツールを通じて段階的に導入可能
#ScalaMatsuri
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4.
NEXT STEPS
今日から始める Eta
Let’s be a Eta expert
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Tour of Eta
● Online tutorials with a sandbox
○ https://tour.eta-lang.org
● The best starting point for Eta newbies
● Focusing on practical use
○ Basics, like syntax and flow control
○ “Haskell-ish” design patterns
○ A little more practical than “Learn you a Haskell”
Eta 公式のオンラインチュートリアル
#ScalaMatsuri
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Eta 公式のオンラインチュートリアル
#ScalaMatsuri
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Japan Haskell User Group (Haskell-jp)
● Since Mar. 2017
● Monthly meetup
○ a.k.a. Mokumoku-kai
○ Newbies are welcome!
○ https://haskell-jp.connpass.com
● Slack / Reddit / Blog
○ https://haskell.jp
日本 Haskell ユーザグループの活動
#ScalaMatsuri
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日本 Haskell ユーザグループの活動
#ScalaMatsuri
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Google Summer of Code
● Ideas List (abr)
○ Better type error messages
○ Eta on Android
○ High-performance Web server with Fibers threading
○ Purity & nullability analysis
● Deadline: 27th Mar. 16:00 UTC
○ https://eta-lang.org/contribute/gsoc/2018
Google Summer of Code の対象プロジェクトに
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5.
SUMMARY
本日のまとめ
What you’ve gotten in 40 mins
#ScalaMatsuri
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Summary
● IO in Haskell
○ Compiler can check side-effect as a type
● Java/Scala interoperation
○ Java monads + type-level GHC extensions
● Strategy for hybrid projects
○ Haskell is for internal pure logic, inside Scala
本日のまとめ
#ScalaMatsuri
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Scala もいいけど Haskell もね！
Happy Haskelling!
Presented by Cheshire Cat (@y_taka_23)
#ScalaMatsuri