コンテキストでの型の操作 do表記を使ってMonadを扱いやすくする Monadの全てのインスタンスに対応させる、型シグネチャ
入門Haskellプログラミング (Will Kurt(著)、株式会社クイープ(監修、翻訳)、翔泳社)のUNIT5(コンテキストでの型の操作)、LESSON 31(do表記を使ってMonadを扱いやすくする)、31.4(練習問題)Q31-3の解答を求めてみる。
コード
type Pizza = (Double, Double)
areaGivenDiameter :: Double -> Double
areaGivenDiameter size = pi * (size/2)^2
costPerInch :: Pizza -> Double
costPerInch (size, cost) = cost / areaGivenDiameter size
comparePizzas :: Pizza -> Pizza -> Pizza
comparePizzas p1 p2 =
let costP1 = costPerInch p1
costP2 = costPerInch p2
in if costP1 < costP2
then p1
else p2
describePizza :: Pizza -> String
describePizza (size, cost) =
let costSqInch = costPerInch (size, cost)
in mconcat ["The ",
show size,
" pizza ",
"is cheaer at ",
show costSqInch,
" per square inch"]
costs :: [Double]
costs = [18, 16]
sizes :: [Double]
sizes = [20, 15]
monadMain :: Monad m => m Double -> m Double -> m String
monadMain sizes costs = do
size1 <- sizes
cost1 <- costs
size2 <- sizes
cost2 <- costs
let pizza1 = (size1, cost1)
let pizza2 = (size2, cost2)
let betterPizza = comparePizzas pizza1 pizza2
return (describePizza betterPizza)
main :: IO ()
main = do
let result = monadMain sizes costs
print (length result)
mapM_ print result
入出力結果(Terminal, Zsh)
% runghc sample3.hs
16
"The 20.0 pizza is cheaer at 5.729577951308232e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 20.0 pizza is cheaer at 5.729577951308232e-2 per square inch"
"The 20.0 pizza is cheaer at 5.729577951308232e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 20.0 pizza is cheaer at 5.729577951308232e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 15.0 pizza is cheaer at 0.10185916357881301 per square inch"
"The 15.0 pizza is cheaer at 9.054147873672269e-2 per square inch"
"The 20.0 pizza is cheaer at 5.729577951308232e-2 per square inch"
"The 20.0 pizza is cheaer at 5.092958178940651e-2 per square inch"
"The 15.0 pizza is cheaer at 9.054147873672269e-2 per square inch"
"The 15.0 pizza is cheaer at 9.054147873672269e-2 per square inch"
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