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當我嘗試在ghci的一些文件加載像putStrLn $ showManyP "%d" 10
它的工作原理,但之後爲什麼這不,當我把它寫在文件 main = putStrLn $ showManyP "%d" 10
作品在ghci中而不是在文件
工作它給這個錯誤
printf.hs:37:19:
Ambiguous type variable `a0' in the constraints:
(Format a0) arising from a use of `showManyP' at printf.hs:37:19-27
(Num a0) arising from the literal `10' at printf.hs:37:34-35
Probable fix: add a type signature that fixes these type variable(s)
In the second argument of `($)', namely `showManyP "%d" 10'
In the expression: putStrLn $ showManyP "%d" 10
In an equation for `main': main = putStrLn $ showManyP "%d" 10
Failed, modules loaded: none.
實際的文件從這裏開始:
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeSynonymInstances #-}
import Data.List (intercalate,isPrefixOf)
class Showable a where
showManyP :: String -> a
instance Showable String where
showManyP str = str
instance (Showable s,Format a) => Showable (a->s) where
showManyP str a = showManyP (format str a)
class Format a where
format :: String -> a -> String
instance Format String where
format str a = replace "%s" str a
instance Format Char where
format str a = replace "%c" str [a]
instance Num a=>Format a where
format str a = replace "%d" str (show a)
replace :: String -> String -> String -> String
replace f str value = intercalate value $ split str f
split :: String -> String -> [String]
split [] f = [[]]
split str [] = [str]
split [email protected](x:xs) f | isPrefixOf f str = [[],drop (length f) str]
| otherwise = let (y:ys) = split xs f
in [x:y] ++ ys
這不完全正確。在已編譯的Haskell模塊中啓用類型默認_is_,相當於聲明'default(Integer,Double)'。然而它不適用於這種情況,因爲'Format'不是標準類。 (請參閱您的第一個鏈接)。然而,GHCi解除了這一限制。 – hammar
你能告訴如何在上面的情況下添加格式的默認聲明..? – Satvik