FreeモナドでDSLを作って、後から実装を変える例

code:hs

-- DSLのInterfaceを定義

data CalcF a

= Add Int a

| Sub Int a

| Mul Int a

| Div Int a

deriving (Functor)

-- Freeモナドを作成

type Calc a = Free CalcF a

-- ヘルパー関数の用意

add, sub, mul, div' :: Int -> Calc ()

add x = liftF (Add x ())

sub x = liftF (Sub x ())

mul x = liftF (Mul x ())

div' x = liftF (Div x ())

-- DSLを使ってコードを書く

program :: Calc ()

program = do

add 10

sub 5

mul 2

div' 2

code:hs

-- 実装1: 四則演算の結果をIntで返す

interpret :: Calc a -> Int -> Int

interpret (Free (Add x next)) state = interpret next (state + x)

interpret (Free (Sub x next)) state = interpret next (state - x)

interpret (Free (Mul x next)) state = interpret next (state * x)

interpret (Pure _) state = state

code:hs

-- 実装2: 四則演算の結果をStringで返す

interpretString :: Calc a -> Int -> String

interpretString (Free (Add x next)) state = interpretString next (state + x)

interpretString (Free (Sub x next)) state = interpretString next (state - x)

interpretString (Free (Mul x next)) state = interpretString next (state * x)

interpretString (Pure _) state = show state

ヘルパー関数は、ヘルパー関数に過ぎず、本質は型を組み合わせて構造を作るとこ

あとは、その構造をどういう風に解釈するか