实例显示haskell [英] instance Show haskell

问题描述

我得到一个数据类型Cake

I got a data type "Cake"

*data Cake = EmptyBox | Food { name :: String
                                , flavors :: [Cake]} deriving( Read, Eq)*

instance Show Cake where
  show cake = prints cake where
   prints (Food name []) = name ++ "\n"

我想打印这样的蛋糕（每个蛋糕有不同的口味）

I want to print cakes like this (each cake got different flavors)

Cake1
  Chocolate
  Nutella
    Strawberry
Cake2
  Chocolate
  Vanilla
Cake3

但是我得到错误，不起作用！

But I got error, doesn't works! How can I do it?

推荐答案

对于测试数据，我使用了：

For test data, I used:

cake1 = Food{name="Cake1"
            ,flavors=[Food{name="Chocolate"
                          ,flavors=[]}
                     ,Food{name="Nutella"
                          ,flavors=[Food{name="Strawberry"
                                        ,flavors=[]}]}]}
cake2 = Food{name="Cake2"
            ,flavors=[Food{name="Chocolate"
                          ,flavors=[]}
                     ,Food{name="Vanilla"
                          ,flavors=[]}]}
cake3 = Food{name="Cake3"
            ,flavors=[]}

并将 Show Cake 写为 unlines ，该函数由类型为 Cake的函数组成>> [String]

and wrote Show Cake as unlines composed with a function that has type Cake -> [String]

instance Show Cake where
  show = unlines . prints where
    prints :: Cake -> [String]
    prints EmptyBox    = []
    prints (Food s []) = [s]  -- a Food with no subflavors 
    prints (Food s fs) = s:concatMap (map ("  "++) . prints) fs

最后一行处理一般情况下的<$名称 s 和风味 fs 通过映射食品 c>在 fs 上打印，然后在每个子列表上映射（++）
$ b

That last line handles the general case of a Food with name s and flavors fs by mapping prints over fs, then mapping (" "++) over each sublist in that map, and concat'ing them together.

("  "++)                             :: String -> String
map ("  "++)                         :: [String] -> [String]
map ("  "++) . prints                :: Cake -> [String]
map (map ("  "++) . prints)          :: [Cake] -> [[String]]
concat . map (map ("  "++) . prints) :: [Cake] -> [String]

concatMap = concat . map
concatMap (map ("  "++) . prints)    :: [Cake] -> [String]

每个连续的flavor都增加了一个额外的缩进级别（这就是（++）是for）。我们可以测试它：

Each successive level of flavors adds an extra level of indentation (that's what the (" "++) is for). We can test it:

TestModule> putStrLn $ concatMap show [cake1, cake2, cake3]
Cake1
  Chocolate
  Nutella
    Strawberry
Cake2
  Chocolate
  Vanilla
Cake3

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