如何使用實例？

Question

錯誤消息：

ho8.hs:17:19: 
    Ambiguous type variable `a0' in the constraints: 
     (PPLetter a0) arising from a use of `ppLetter' at ho8.hs:17:19-26 
     (Num a0) arising from the literal `3' at ho8.hs:17:28 
    Probable fix: add a type signature that fixes these type variable(s) 
    In the first argument of `print', namely `(ppLetter 3)' 
    In the expression: print (ppLetter 3) 
    In the expression: do { print (ppLetter 3) } 
Failed, modules loaded: none. 

源代碼：

module Main where 
import Data.List(nub) 

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

class PPLetter a where 
ppLetter :: a -> Doc 

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

instance PPLetter Char where 
ppLetter = PP.char 


main = do {print (ppLetter 3);} 

Answer 1

爲您更新的問題，更換

main = do {print (ppLetter 3);} 

與

main = do {print (ppLetter (3 :: Int));} 

Answer 2

顯示約爲::一個 - >字符串

原來的值成一個字符串，但是並不打印向屏幕。

做需要一個IO動作實際打印字符串來篩選，通常

putStr :: String -> IO() 

這樣試試：

main = do { putStr (show (ppLetter 3)) } 

更簡潔打印結合了這兩種：

main = do { print (ppLetter 3) }