为什么我从GHCi得到这个警告？

Question

我得到一个奇怪的警告时，模式匹配，但只启用OverloadedStrings当...

$ ghci -Wall 
GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Prelude> let f x = case (x :: [String]) of {[""] -> "root"; ["product", _] -> "product"; _ -> "unknown"} 
Prelude> :q 
Leaving GHCi. 
$ ghci -Wall -XOverloadedStrings 
GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Prelude> let f x = case (x :: [String]) of {[""] -> "root"; ["product", _] -> "product"; _ -> "unknown"} 

<interactive>:1:10: 
    Warning: Pattern match(es) are overlapped 
      In a case alternative: [""] -> ... 
Prelude> let g x = case (x :: [String]) of {[] -> "root"; ["product", _] -> "product"; _ -> "unknown"} 
Prelude> let h x = case (x :: [String]) of {["oops"] -> "root"; ["product", _] -> "product"; _ -> "unknown"} 
Prelude> :q 
Leaving GHCi. 

我不明白为什么我得到警告为f与OverloadedStrings，特别是因为我不在没有OverloadedStrings的情况下获得f的警告，也不会收到g或h的警告，这些警告仅在第一种模式（在所有情况下仅匹配单个特定值）与f不同。

假设这不是GHC中的错误，我错过了什么？

Answer 1

这里有一个稍微简单在GHC 6.12.3中显示相同问题的示例：

f :: String -> Bool 
f "" = True 
f "a" = False 

g :: String -> Bool 
g "" = True 
g "aa" = False 

只有g得到-XOverloadedStrings的重叠警告。我认为这必须是一个错误。

Answer 2

编辑：基本上，你想要这个（匹配来自(IsString b) => b转换回[Char]之后，但匹配是一致的类型进行）：

f :: [String] -> String 
f = matchf 

matchf :: (Show b, IsString a, Eq a, IsString b) => [a] -> b 
matchf x = case x of [""] -> "root"; ["product", _] -> "product"; _ -> "unknown" 

否则GHC发出警告匹配"" :: String到"" :: (Data.String.IsString t) => t（文字）。这将给予是有趣的，看看为什么字面""正确默认字符串（可能是一个错误？）：

Prelude> show ("" :: (Data.String.IsString t) => t) 

<interactive>:1:0: 
    Warning: Defaulting the following constraint(s) to type `String' 

---

你的字符串必须是派生的EQ模式匹配-XOverloadedStrings工作。字符串仍然只是[Char]与-XOverloadedStrings，但字符串文字不是。

另一种方式来做到这一点，而不会触发报警：

test.hs：

import GHC.Exts(IsString(..)) 

newtype OString = OString String deriving (Eq, Show) 
instance IsString OString where fromString = OString 

f :: [OString] -> OString 
f x = case (x :: [OString]) of {[""] -> "root"; ["product", _] -> "product"; _ -> "unknown"} 

运行：

$ ghci -Wall -XOverloadedStrings 
GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Prelude> :l test.hs 
[1 of 1] Compiling Main    (test.hs, interpreted) 
Ok, modules loaded: Main. 
*Main> f [] 
OString "unknown" 
*Main> f [""] 
OString "root" 
*Main> f ["product"] 
OString "unknown" 
*Main> f ["product", "x"] 
OString "product" 

来源：http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/type-class-extensions.html#overloaded-strings