在Haskell中做什麼顯示和渲染？

Question

例如，在這樣的代碼：

module Main(letters) where 

import Data.List(nub) 

import qualified Text.PrettyPrint.HughesPJ as PP 
import Text.PrettyPrint.HughesPJ(Doc,text,int,(<>),(<+>),($+$),render) 

data Prop a = 
    LetterP a 
    | AndP (Prop a) (Prop a) 
deriving Eq 

class PPLetter a where 
    ppLetter :: a -> Doc 

instance PPLetter Int where 
    ppLetter a = text ("p"++show a) 

instance PPLetter Char where 
    ppLetter = PP.char 

instance PPLetter a => PPLetter [a] where 
    ppLetter = PP.hcat . (map ppLetter) 

class PP a where 
    pp :: a -> Doc 

instance PP Bool where 
    pp True = text "True" 
    pp False = text "False" 

parens n (term@(LetterP _)) = pp term 

instance PPLetter a => PP(Prop a) where 
    pp (LetterP a) = ppLetter a 
    pp (AndP x y) = PP.sep [ parens 4 x, text "/\\", parens 4 y] 

instance PPLetter a => Show (Prop a) where 
    show x = render (pp x) 

main = do 
    let p = LetterP 1 
    print p 

Answer 1

1. The Show class，及其方法，show，將值轉換爲字符串。

2. render功能是Text.PrettyPrint模塊的一部分，該模塊將值轉換爲精美打印的字符串（例如，使用佈局，縮進和嵌套）。

他們一起用於pretty printing