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为什么我要从GHCI那里得到这个警告?

overloaded-strings case-statement ghc warnings haskell

dave4420 · 技术社区 · 14 年前

当模式匹配时,我会收到一个奇怪的警告,但只有当启用重载字符串时…

$ 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 f 使用重载字符串,尤其是因为我没有收到 F 如果没有重载字符串,也不会收到 g 或 h 不同于 F 仅在第一个模式中(在所有情况下,仅匹配单个特定值)。

假设这不是GHC中的错误,我遗漏了什么?

2 回复 | 直到 14 年前

Travis Brown 14 年前

下面是一个稍微简单一点的例子,它在GHC 6.12.3中显示了相同的问题:

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

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

只有 g 获取重叠警告 -XOverloadedStrings . 我想这一定是个虫子。

vls 14 年前

编辑:基本上你想要这个(匹配后转换回 (IsString b) => b 进入之内 [Char] 但匹配是在一致的类型中完成的):

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会发出匹配警告 "" :: String 到 "" :: (Data.String.IsString t) => t (文字)。找出原因(可能是一个错误)会很有趣。考虑到这个字面意思 "" 正确默认为字符串:

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

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

您的字符串必须派生eq才能与-xOverloadedStrings进行模式匹配。字符串仍然是带有-XoverloadedStrings的[char],但字符串文本不是。

另一种不触发警告的方法是:

试验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"}

运行它:

$ 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"

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