はじめに
- 前回
- Elixirを使いはじめてだいたい1年くらいがたちました
- すごいHaskellたのしく学ぼう!という本を2015年に買って、一通り読んだあとずっと本棚にしまわれていたままでした
- 久しぶりに引っ張り出して読んでみると、こんなに愉快な内容だったけ！？ という感想を持ちました
- Elixirで関数プログラミングにだいぶ慣れたので、ユーモアの部分を楽しむ余裕ができたのだとおもいます
- 少しずつ読み進めながら、興味が向いたところだけElixirで書き換えてみたりして理解を深めていきたいとおもいます
- 今回はHello 再帰!
maximum
- リストの中から一番でっかいやつを返します
baby.hs
maximum' [] = error "maximum of empty list!"
maximum' [x] = x
maximum' (x:xs) = max x (maximum' xs)
Prelude> maximum' [2,5,1]
5
awesome.exs
defmodule Awesome do
def maximum([]), do: raise "maximum of empty list!"
def maximum([x]), do: x
def maximum([x|xs]), do: max(x, maximum(xs))
end
iex> Awesome.maximum([2,5,1])
5
replicate n x
- n個の要素がxのリストを返します
baby.hs
replicate' n x
| n <= 0 = []
| otherwise = x : replicate' (n-1) x
Prelude> replicate' 10 3
[3,3,3,3,3,3,3,3,3,3]
awesome.exs
defmodule Awesome do
def replicate(n, _) when n <= 0, do: []
def replicate(n, x), do: [x | replicate(n - 1, x)]
end
iex> Awesome.replicate(10, 3)
[3, 3, 3, 3, 3, 3, 3, 3, 3, 3]
take n [a]
- リストからn個取り出します
baby.hs
take' n _
| n <= 0 = []
take' _ [] = []
take' n (x:xs) = x : take' (n-1) xs
Prelude> take' 2 [1,2,3,4,5]
[1,2]
awesome.exs
defmodule Awesome do
def take(_, n) when n <= 0, do: []
def take([], _), do: []
def take([x | xs], n), do: [x | take(xs, n - 1)]
end
iex> Awesome.take([1,2,3,4,5],2)
[1, 2]
- 引数の順は、Enum.take/2とあわせました
reverse
- リストの中身をひっくり返します
baby.hs
reverse' [] = []
reverse' (x:xs) = reverse' xs ++ [x]
Prelude> reverse' [1,2,3]
[3,2,1]
awesome.exs
defmodule Awesome do
def reverse([]), do: []
def reverse([x | xs]), do: reverse(xs) ++ [x]
end
iex> Awesome.reverse([1,2,3])
[3, 2, 1]
repeat
- 無限リストをつくります
baby.hs
repeat' x = x : repeat' x
Prelude> take' 10 (repeat' 3)
[3,3,3,3,3,3,3,3,3,3]
awesome.exs
defmodule Awesome do
def repeat(x), do: Stream.cycle([x])
end
iex> Awesome.repeat(3) |> Enum.take(10)
[3, 3, 3, 3, 3, 3, 3, 3, 3, 3]
zip
- zipします
baby.hs
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y) : zip' xs ys
Prelude> zip' [1,2,3] ['a', 'b']
[(1,'a'),(2,'b')]
awesome.exs
defmodule Awesome do
def zip(_, []), do: []
def zip([], _), do: []
def zip([x | xs], [y | ys]), do: [{x,y} | zip(xs, ys)]
end
iex> Awesome.zip([1,2,3], ["a", "b"])
[{1, "a"}, {2, "b"}]
elem
- 値がリストに含まれるかどうか
baby.hs
elem' a [] = False
elem' a (x:xs)
| a == x = True
| otherwise = a `elem'` xs
Prelude> 3 `elem` [1,2,3]
True
Prelude> -1 `elem` [1,2,3]
False
awesome.exs
defmodule Awesome do
def member?([], _), do: false
def member?([x | _], element) when x == element, do: true
def member?([_ | xs], element), do: member?(xs, element)
end
iex> Awesome.member?([1,2,3], 3)
true
iex> Awesome.member?([1,2,3], -1)
false
quicksort
- クイックソートです
baby.hs
quicksort [] = []
quicksort (x:xs) =
let smallerOrEqual = [a | a <- xs, a <= x]
larger = [a | a <-xs, a > x]
in quicksort smallerOrEqual ++ [x] ++ quicksort larger
Prelude> quicksort [10,2,5,3,1,6,7,4,2,3,4,8,9]
[1,2,2,3,3,4,4,5,6,7,8,9,10]
awesome.exs
defmodule Awesome do
def quicksort([]), do: []
def quicksort([x | xs]) do
smaller_or_equal = (for a <- xs, a <= x, do: a)
larger = (for a <- xs, a > x, do: a)
quicksort(smaller_or_equal) ++ [x] ++ quicksort(larger)
end
end
iex> Awesome.quicksort([10,2,5,3,1,6,7,4,2,3,4,8,9])
[1, 2, 2, 3, 3, 4, 4, 5, 6, 7, 8, 9, 10]
