Question

在我加载像putStrLn $ showManyP "%d" 10这样的文件后在ghci中尝试一下它工作，但为什么这在我在文件中写它时不起作用 main = putStrLn $ showManyP "%d" 10

它给出了这个错误

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.

实际文件从这里开始：

{-# 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 str@(x:xs) f | isPrefixOf f str = [[],drop (length f) str]
                   | otherwise = let (y:ys) = split xs f
                                 in [x:y] ++ ys

Answer 1

在ghc中，当您输入类似10的数字常量时，它可以是Num的实例的任何类型。如果没有其他类型约束，则它是未确定的实例，并且您必须提供特定类型;即(10 :: Int)。 Ghci是交互式的，必须向数字添加类型会很痛苦，所以它可以通过假设在没有其他类型约束的情况下看起来像整数的类型为Integer来帮助你。这在GHC用户指南2.4.5. Type defaulting in GHCi

中有解释

根据“2010 Haskell报告”，在4.3.4 Ambiguous Types, and Defaults for Overloaded Numeric Operations中，有一个default关键字，允许您在已编译的模块中利用此行为。

在ghci中工作但不在文件中

1 个答案: