[英]GADTs, TypeFamilies type inference failure when implementing “mixins”
[英]TypeFamilies or GADTs suddenly breaks the valid code
我有非常无辜的代码
data Config = Config
{ cInts :: [Int]
, cStrings :: [String] }
instance Semigroup Config where
c1 <> c2 = Config
{ cInts = andCombiner cInts
, cStrings = andCombiner cStrings }
where
andCombiner field = field c1 <> field c2
它编译并正常工作。 但是,如果我添加TypeFamilies
或GADTs
扩展,我会看到非常奇怪的错误:
.../Main.hs:19:22: error:
• Couldn't match type ‘Int’ with ‘[Char]’
Expected type: [String]
Actual type: [Int]
• In the ‘cStrings’ field of a record
In the expression:
Config {cInts = andCombiner cInts, cStrings = andCombiner cStrings}
In an equation for ‘<>’:
c1 <> c2
= Config
{cInts = andCombiner cInts, cStrings = andCombiner cStrings}
where
andCombiner field = field c1 <> field c2
|
19 | , cStrings = andCombiner cStrings }
| ^^^^^^^^^^^^^^^^^^^^
.../Main.hs:19:34: error:
• Couldn't match type ‘[Char]’ with ‘Int’
Expected type: Config -> [Int]
Actual type: Config -> [String]
• In the first argument of ‘andCombiner’, namely ‘cStrings’
In the ‘cStrings’ field of a record
In the expression:
Config {cInts = andCombiner cInts, cStrings = andCombiner cStrings}
|
19 | , cStrings = andCombiner cStrings }
| ^^^^^^^^
这个编译器错误的原因是什么?
这是由于-XMonoLocalBinds
-XGADTs
和-XTypeFamilies
暗示的。 您可以通过向andCombiner
添加类型签名(或通过启用-XNoMonoLocalBinds
,尽管我不建议这样做)来再次编译代码:
instance Semigroup Config where
c1 <> c2 = Config
{ cInts = andCombiner cInts
, cStrings = andCombiner cStrings }
where
andCombiner :: Semigroup a => (Config -> a) -> a
andCombiner field = field c1 <> field c2
使用我链接的GHC文档中的术语, andCombiner
并未完全概括,因为它提到了未关闭或导入的c1
和c2
。
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