重叠模式匹配
我有以下代码：
test :: String -> Bool
test "g" = True
test "global" = True
test _ = False
当我将其加载到 GHCi (7.0.3) 中时，我得到：
Warning: Pattern match(es) are overlapped
In an equation for `test': test "g" = ...
这是一个错误还是我在这里遗漏了一些东西？
以下保留：
test "" == False
test "g" == True
test "gl" == False
test "global" == True
test "globalx" == False
更新：
我正在使用 {-# LANGUAGE OverloadedStrings #-}。
I have the following code:
test :: String -> Bool
test "g" = True
test "global" = True
test _ = False
When I load it into GHCi (7.0.3), I get:
Warning: Pattern match(es) are overlapped
In an equation for `test': test "g" = ...
Is this a bug or am I missing something here?
The following hold:
test "" == False
test "g" == True
test "gl" == False
test "global" == True
test "globalx" == False
UPDATE:
I am using {-# LANGUAGE OverloadedStrings #-}.
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这是 GHC bug #5117，由使用
OverloadedStrings扩展引起。它应该在 GHC 7.2 中修复。作为解决方法，您可以使用
{-# LANGUAGE NoOverloadedStrings #-}关闭模块的OverloadedStrings，或使用{-# OPTIONS_GHC - 关闭警告fno-warn-overlapping-patterns #-}。或者直接忽略它:)This is GHC bug #5117, arising from the use of the
OverloadedStringsextension. It should be fixed in GHC 7.2.As a workaround, you could turn off
OverloadedStringsfor the module with{-# LANGUAGE NoOverloadedStrings #-}, or turn off the warning with{-# OPTIONS_GHC -fno-warn-overlapping-patterns #-}. Or just ignore it :)您是否打开了
OverloadedStrings？如果我没记错的话，这会导致“虚假”重叠模式警告，因为在这种情况下，不清楚“g”和“global”是否相互排斥。Have you turned on
OverloadedStrings? If I remember correctly, that causes 'spurious' overlapping patterns warnings, because in that case it's not clear that e.g. "g" and "global" are mutually exclusive.