哈斯克尔。解析命令行参数

Question

我正在学习Haskell编程语言。现在我正在学习IO。我想编写简单的命令行util。 util获取两个文件（JSON或YAML），对它们进行比较并输出比较结果（JSON，YAML，纯文本）。

我认为，我的uter界面是：

my_util "path to first file" "path to second file" -- format JSON. 

我决定使用cmdargs。但是现在，我的界面显示了：

my_util -f="path to first file" -s="path to second file" -- format JSON.

我做了什么。

data Gendiff = Gendiff
    {first_file ::  String
    ,second_file :: String
    ,format :: String
    }
    deriving (Data,Typeable,Show,Eq)
 
gendiff = Gendiff
    {first_file  = def &=  name "f" &= typDir  &= help "path to first file to compare"  &= typ "String"
    ,second_file = def &=  name "s" &=  typDir  &= help  "path to second file to compare" &= typ "String"
    ,format =  &= opt "JSON" &= help "set format of output"  &= typ "String"
    } &=
    verbosity &=
    help "Compares two configuration files and shows a difference." &=
    summary " Gendiff v0.0.1, (C) dosart"
 
main = print =<< cmdArgs gendif

我如何制作第一个变体？如何设置格式标志默认值？

Answer 1

我猜您会使用 args 。这样的事情：

data Gendiff = Gendiff
    { format :: String
    , files :: [String]
    }

gendiff = Gendiff
    { format = def &= -- same as before
    , files = def &= args
    } &= -- same as before

main = do
    args <- cmdArgs gendiff
    case args of
        Gendiff { files = [first, second] } -> -- success path
        _ -> -- print an error