Haskell 中的列表输出格式？

Question

我在尝试在 Haskell 中格式化我自己类型的列表的输出时遇到问题。

我想要这样的东西：

Make  | Model | Years(this is a list)    <- this would be the headers if you like
-------------------
Item1 | Item1 | Item1s,Item1s           
Item2 | Item2 | Item2s,Items2,Items2

^ 这将是从我的 String String [Int] 类型加载的数据。

我该如何在 Haskell 中做到这一点？

Answer 1

一般来说，我们使用“漂亮的打印”库来完成漂亮的格式化输出。您应该知道的标准是 Text.PrettyPrint。给定一个数据类型，您可以遍历该类型，构建格式良好的文档。

一个例子：

import Text.PrettyPrint
import Data.List

-- a type for records
data T = T { make  :: String
           , model :: String
           , years :: [Int] }
    deriving Show

-- test data
test =
    [ T "foo" "avenger" [1990, 1992]
    , T "bar" "eagle"   [1980, 1982]
    ]

-- print lists of records: a header, then each row
draw :: [T] -> Doc
draw xs =
    text "Make\t|\tModel\t|\tYear"
   $+$
    vcat (map row xs)
 where
    -- print a row
    row t = foldl1 (<|>) [ text (make t)
                         , text (model t)
                         , foldl1 (<^>) (map int (years t))
                         ]

-- helpers
x <|> y = x <> text "\t|\t" <> y
x <^> y = x <> text "," <+> y

测试：

main = putStrLn (render (draw test))

结果：

Make    |   Model   |   Year
foo     |   avenger |   1990, 1992
bar     |   eagle   |   1980, 1982

快速编写漂亮的打印机的能力是一项非常有用的技能。

Answer 2

这是一个通用的表生成器。它计算列宽以适合最宽的行。 ColDesc 类型允许您为每列指定标题对齐方式、标题字符串、数据对齐方式以及格式化数据的函数。

import Data.List (transpose, intercalate)

-- a type for records
data T = T { make  :: String
           , model :: String
           , years :: [Int] }
    deriving Show

-- a type for fill functions
type Filler = Int -> String -> String

-- a type for describing table columns
data ColDesc t = ColDesc { colTitleFill :: Filler
                         , colTitle     :: String
                         , colValueFill :: Filler
                         , colValue     :: t -> String
                         }

-- test data
test =
    [ T "foo" "avenger" [1990, 1992]
    , T "bar" "eagle"   [1980, 1982, 1983]
    ]

-- functions that fill a string (s) to a given width (n) by adding pad
-- character (c) to align left, right, or center
fillLeft c n s = s ++ replicate (n - length s) c
fillRight c n s = replicate (n - length s) c ++ s
fillCenter c n s = replicate l c ++ s ++ replicate r c
    where x = n - length s
          l = x `div` 2
          r = x - l

-- functions that fill with spaces
left = fillLeft ' '
right = fillRight ' '
center = fillCenter ' '

-- converts a list of items into a table according to a list
-- of column descriptors
showTable :: [ColDesc t] -> [t] -> String
showTable cs ts =
    let header = map colTitle cs
        rows = [[colValue c t | c <- cs] | t <- ts]
        widths = [maximum $ map length col | col <- transpose $ header : rows]
        separator = intercalate "-+-" [replicate width '-' | width <- widths]
        fillCols fill cols = intercalate " | " [fill c width col | (c, width, col) <- zip3 cs widths cols]
    in
        unlines $ fillCols colTitleFill header : separator : map (fillCols colValueFill) rows

运行：

putStrLn $ showTable [ ColDesc center "Make"  left  make
                     , ColDesc center "Model" left  model
                     , ColDesc center "Year"  right (intercalate ", " . map show . years)
                     ] test

结果：

Make |  Model  |       Year      
-----+---------+-----------------
foo  | avenger |       1990, 1992
bar  | eagle   | 1980, 1982, 1983

Answer 3

像这样的东西吗？

import Data.List (intercalate)
data Foo = Foo String String [Int]

fooToLine :: Foo -> String
fooToLine (Foo a b cs) = a ++ " | " ++ b ++ " | " ++ intercalate ", " (map show cs)

现在，你可以做

>>> fooToLine (Foo "Hello" "World" [1, 2, 3])
"Hello | World | 1, 2, 3"