幻影类型使图案匹配无可辩驳，但似乎在内部不起作用。
请查看代码。我相信使用Phantom类型使模式匹配不可撤销，因此在MonadFail实例中无需。
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# OPTIONS_GHC -Wall #-}
{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}
data Type = I | S
data T t where
TI :: Int -> T 'I
TS :: String -> T 'S
ti :: T 'I
ti = TI 42
test :: Monad m => m Int
test = do
(TI v) <- return ti
return v
但是我遇到了这个错误：
• Could not deduce (MonadFail m)
arising from a do statement
with the failable pattern ‘(TI v)’
这种方法怎么了？
我用GHC 9.0.2和8.10.4进行了检查。
顺便说一句，即使在存在-wincomplete-uni-patterns选项的情况下，在中匹配它也不会产生任何警告。
Please look at the code. I believe using phantom type makes the pattern matching irrefutable so there is no need in MonadFail instance.
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# OPTIONS_GHC -Wall #-}
{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}
data Type = I | S
data T t where
TI :: Int -> T 'I
TS :: String -> T 'S
ti :: T 'I
ti = TI 42
test :: Monad m => m Int
test = do
(TI v) <- return ti
return v
But I am getting this error:
• Could not deduce (MonadFail m)
arising from a do statement
with the failable pattern ‘(TI v)’
What's wrong with this approach?
I checked this with ghc 9.0.2 and 8.10.4.
By the way, matching it in let does not produce any warning even in the presence of -Wincomplete-uni-patterns option.
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您的方法没有错；它应该按照您的期望工作。在GHC中，它目前尚未： nofollow noreferrer“考虑到类型的信息（＃25085）
在修复它之前，您可以通过编写
〜（ti v）＆lt; - 返回ti而不是（ti V） ）＆lt; - 返回ti。There's nothing wrong with your approach; it should work as you expect it to. It's a known bug in GHC that it currently doesn't: MonadFail exhaustivity checking does not take into account type information (#25085)
Until they fix it, you can work around it by writing
~(TI v) <- return tiinstead of(TI v) <- return ti.