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数字の文字列をread使って数字列に変換する

やり方

read . returnを使って型を指定すれば良い

Prelude> let n = "123456789" :: String
Prelude> n
"123456789"
Prelude> map return n :: [String]
["1","2","3","4","5","6","7","8","9"]
Prelude> map (read . return) n :: [Int]
[1,2,3,4,5,6,7,8,9]
Prelude> map (read . return) n :: [Float]
[1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0]

[String]の場合には以下のようにmapしてあげれば良い

Prelude> let parse str = map (read . return) str :: [Int]
Prelude> :t parse
parse :: [Char] -> [Int]
Prelude> map parse ["1234","5678","9012"]
[[1,2,3,4],[5,6,7,8],[9,0,1,2]]
Prelude> map product $ map parse ["1234","5678","9012"]
[24,1680,0]

ちょっと詳しく

大事なのはこの型指定

Prelude> map (read . return) n :: [Int]

returnをリスト(モナド)への変換操作として用いており、
さらにreadの返り値としてIntを同時に指定している

分解するとこうなる

Prelude> let x = return "100" :: [String]
Prelude> x
["100"]
Prelude> map read x :: [Int]
[100]

当然リスト以外を使うことも出来て、
例えば文字列をMaybe Intに変換したければfmapを使って以下のように書ける

Prelude> fmap read $ return "100" :: Maybe Int
Just 100
Prelude> let x = return "100" :: Maybe String
Prelude> fmap read x :: Maybe Int
Just 100

使えるシーン

Problem 8 - Project Euler

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