Haskell：列表理解

Question

如何在不重复 (c !! x) 的情况下编写此代码，其中 x = [0..7]，使用列表理解更好，但我不知道如何编写它

show (EOBoard f c r) = "EOBoard\n" ++ "Foundations "
   ++ show f ++ "\n" ++ "Columns \n" 
   ++ show (c!!0) ++ "\n" ++ show (c!!1) ++ "\n"
   ++ show (c!!2) ++ "\n" ++ show (c!!3) ++ "\n"
   ++ show (c!!4) ++ "\n" ++ show (c!!5) ++ "\n"
   ++ show (c!!6) ++ "\n" ++ show (c!!7) ++ "\n"
   ++ "Reserves " ++ show r

Answer 1

让我们首先摆脱所有这些手动换行符。

import Data.List

show (EOBoard f c r) = intercalate "\n" $
  [ "EOBoard"
  , "Foundations " ++ show f
  , "Columns" 
  , show (c!!0)
  , show (c!!1)
  , show (c!!2)
  , show (c!!3)
  , show (c!!4)
  , show (c!!5)
  , show (c!!6)
  , show (c!!7)
  , "Reserves " ++ show r]

注意：如果您也想在末尾换行，请使用unlines 而不是intercalate。

正如您所注意到的，有一个相当重复的部分。此外，那些!! 应用程序有点贵。列表推导解决了这两个问题，但我会改用 map。

show (EOBoard f c r) = intercalate "\n" $
  [ "EOBoard"
  , "Foundations " ++ show f
  , "Columns" ] ++
  map show c ++
  ["Reserves " ++ show r]

（如果您愿意，map show c 可以替换为 [show x | x <- c]。）

还有一些有趣的东西； Show 真的不适合漂亮的打印。 show 不应插入换行符。您可能实际上想编写一个自定义的漂亮打印函数。

Answer 2

你写的相当于

show (EOBoard f c r) = "EOBoard\n" ++ "Foundations "
   ++ show f ++ "\n" ++ "Columns \n" ++
   ++ concat [ show s ++ "\n" | s <- take 8 c]
   ++ "\n" ++ "Reserves " ++ show r

相当于

show (EOBoard f c r) = "EOBoard\n" ++ "Foundations "
   ++ show f ++ "\n" ++ "Columns \n" ++
   ++ [ ch | s <- take 8 c, ch <- show s ++ "\n" ]
   ++ "\n" ++ "Reserves " ++ show r

或者，使用concat more 而不是内联它，它相当于

show (EOBoard f c r) = concat (
   [ "EOBoard\n", "Foundations ", show f, "\n", "Columns \n" ]
   ++ [ show s ++ "\n" | s <- take 8 c]
   ++ ["\n" , "Reserves " , show r] )

通常使用$ 运算符编写，如

show (EOBoard f c r) = concat $
   [ "EOBoard\n", "Foundations ", show f, "\n", "Columns \n" ]
   ++ [ show s ++ "\n" | s <- take 8 c]
   ++ ["\n" , "Reserves " , show r]