Question

我通过Learn your Haskell上的教程获得了成功我问自己为什么作者使用列表作为已实现的拉链的第二种类型？

以下是相关代码：

func GetKeys(filename string) {
    var record []string
    var err error
    file, err := os.Open(filename)
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()
    // Create a new reader.
    reader := csv.NewReader(bufio.NewReader(file))
    for {
        record, err = reader.Read()
        // Stop at EOF.
        if err == io.EOF {
            break
        }
        fmt.Printf("%v", record) // works fine
        fmt.Printf("%v\n", record[0]) // works fine too
    }
    fmt.Printf("%v\n", record) // returns empty slice
}

func main() {
    GetKeys("credentials.csv")
}

您可以使用的命令：

type Name = String
type Data = String
data FSItem = File Name Data
    | Folder Name [FSItem]
    deriving (Show)

data FSCrumb = FSCrumb Name [FSItem] [FSItem]
    deriving (Show)
type FSZipper = (FSItem, [FSCrumb])

-- -------------------------------------------------------------------------------
-- Some other code he uses
-- -------------------------------------------------------------------------------

fsUp :: FSZipper -> FSZipper
fsUp (item, FSCrumb name ls rs : bs) = (Folder name (ls ++ [item] ++ rs), bs)

fsTo :: Name -> FSZipper -> FSZipper
fsTo name (Folder folderName items, bs) =
    let (ls, item:rs) = break (nameIs name) items
    in  (item, FSCrumb folderName ls rs:bs)

nameIs :: Name -> FSItem -> Bool
nameIs name (Folder folderName _) = name == folderName
nameIs name (File   fileName   _) = name == fileName

x -: f = f x

-- -------------------------------------------------------------------------------
-- Example to work on
-- -------------------------------------------------------------------------------

myDisk :: FSItem  
myDisk = 
    Folder "root"   
        [ File "goat_yelling_like_man.wmv" "baaaaaa"  
        , File "pope_time.avi" "god bless"  
        , Folder "pics"  
            [ File "ape_throwing_up.jpg" "bleargh"  
            , File "watermelon_smash.gif" "smash!!"  
            , File "skull_man(scary).bmp" "Yikes!"  
            ]  
        , File "dijon_poupon.doc" "best mustard"  
        , Folder "programs"  
            [ File "fartwizard.exe" "10gotofart"  
            , File "owl_bandit.dmg" "mov eax, h00t"  
            , File "not_a_virus.exe" "really not a virus"  
            , Folder "source code"  
                [ File "best_hs_prog.hs" "main = print (fix error)"  
                , File "random.hs" "main = print 4"  
                ]  
            ]  
        ]

无论我做什么，我总是得到一个只有一个项目的列表，所以只使用*Filesystem> let newFocus1 = (myDisk,[]) -: fsTo "programs" -: fsTo "source code" *Filesystem> let newFocus2 = (myDisk,[]) -: fsTo "pics" -: fsTo "ape_throwing_up.jpg"而不是FSCrumb会更好吗？

Answer 1

问题很简单，就是用来获取面包屑数量的方法。一种简单（正确）的方法是：

numberOfBreadcrumbs :: FSZipper -> Int
numberOfBreadcrumbs (_, breadcrumbs) = length breadcrumbs

为什么第二种拉链类型是数据列表而不是纯数据？

1 个答案: