関数型プログラミングの基礎 ファーストクラス関数 引数としての関数 引数としてのラムダ式、カスタムソート、タプルのリスト、組み込み関数、Data、Listモジュール、sortBy関数
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 4(ファーストクラス関数)、4.1(引数としての関数)、引数としてのラムダ式のクイックチェック4-1、カスタムソートのクイックチェック4-2の解答を求めてみる。
コード
import Data.List
names :: [([Char], [Char])]
names = [
("Ian", "Curtis"),
("Bernard", "Sumner"),
("Peter", "Hook"),
("Stephen", "Morris"),
("Ian", "Curtis"),
("Bernard", "Hook")
]
compareLastNames :: (Ord b, Ord a) => (a, b) -> (a, b) -> Ordering
compareLastNames name1 name2 =
let
lastName1 = snd name1
lastName2 = snd name2
in
if lastName1 > lastName2
then GT
else
if lastName1 < lastName2
then LT
else
let
firstName1 = fst name1
firstName2 = fst name2
in
if firstName1 > firstName2
then GT
else
if firstName1 < firstName2
then LT
else EQ
入出力結果(Terminal, Zsh)
% ghci
GHCi, version 8.10.4: https://www.haskell.org/ghc/ :? for help
macro 'doc' overwrites builtin command. Use ':def!' to overwrite.
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
Loaded GHCi configuration from /.../.ghc/ghci.conf
Prelude
λ> :load sample1.hs
[1 of 1] Compiling Main ( sample1.hs, interpreted )
Ok, one module loaded.
(0.03 secs,)
*Main
λ> ifEven (\x -> x*3) 0
0
it :: Integral a => a
(0.04 secs, 62,424 bytes)
*Main
λ> ifEven (\x -> x*3) 1
1
it :: Integral a => a
(0.01 secs, 60,008 bytes)
*Main
λ> ifEven (\x -> x*3) 2
6
it :: Integral a => a
(0.00 secs, 60,128 bytes)
*Main
λ> ifEven (\x -> x*3) -1
<interactive>:5:1: error:
• Non type-variable argument in the constraint: Num (a -> a)
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. (Integral a, Num (a -> a)) => a -> a
(0.00 secs,)
*Main
λ> ifEven (\x -> x*3) (-1)
-1
it :: Integral a => a
(0.00 secs, 60,832 bytes)
*Main
λ> ifEven (\x -> x*3) (-2)
-6
it :: Integral a => a
(0.00 secs, 60,952 bytes)
*Main
λ> :load sample2
[1 of 1] Compiling Main ( sample2.hs, interpreted )
Ok, one module loaded.
(0.01 secs,)
*Main
λ> names
[("Ian","Curtis"),("Bernard","Sumner"),("Peter","Hook"),("Stephen","Morris"),("Ian","Curtis"),("Bernard","Hook")]
it :: [([Char], [Char])]
(0.00 secs, 150,704 bytes)
*Main
λ> sort
sort sortBy sortOn
λ> sortBy compareLastNames names
[("Ian","Curtis"),("Ian","Curtis"),("Bernard","Hook"),("Peter","Hook"),("Stephen","Morris"),("Bernard","Sumner")]
it :: [([Char], [Char])]
(0.02 secs, 149,936 bytes)
*Main
λ> :q
Leaving GHCi.
%