関数型プログラミングの基礎 高階関数 filter関数とlength関数によるelem関数の実装
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UNIT1(関数型プログラミングの基礎)、LESSON 9(高階関数)、9.6(練習問題)Q9-1の解答を求めてみる。
コード
lesson/app/Main.hs
module Main where
import Lib (myElem)
main :: IO ()
main = do
mapM_ print $
myElem 5
<$> [[], [1], [5], [1, 5], [5, 1], [1 .. 5], [1 .. 10], [6 .. 10]]
lesson/src/Lib.hs
module Lib
( myElem,
)
where
-- myElem :: (Foldable t, Eq a) => a -> t a -> Bool
myElem :: Eq a => a -> [a] -> Bool
myElem x xs = length (filter (== x) xs) /= 0
入出力結果(Terminal, Zsh)
% stack build
lesson-0.1.0.0: unregistering (local file changes: app/Main.hs)
lesson> build (lib + exe)
Preprocessing library for lesson-0.1.0.0..
Building library for lesson-0.1.0.0..
Preprocessing executable 'lesson-exe' for lesson-0.1.0.0..
Building executable 'lesson-exe' for lesson-0.1.0.0..
[2 of 2] Compiling Main
Linking .stack-work/dist/x86_64-osx/Cabal-3.4.1.0/build/lesson-exe/lesson-exe ...
lesson> copy/register
Installing library in /Users/…/lesson/.stack-work/install/x86_64-osx/494c7f11ebc566ca0dd51f0cf6ed1f4268c15f68d59cb48ce0d42f2bf991d579/9.0.2/lib/x86_64-osx-ghc-9.0.2/lesson-0.1.0.0-CIOIkpTgcfQI9akDJnwnSd
Installing executable lesson-exe in /Users/…/lesson/.stack-work/install/x86_64-osx/494c7f11ebc566ca0dd51f0cf6ed1f4268c15f68d59cb48ce0d42f2bf991d579/9.0.2/bin
Registering library for lesson-0.1.0.0..
% stack exec lesson-exe
False
False
True
True
True
True
True
False
%