[英]Top-level OverloadedLists literal
我有一个用于重构练习的测试套件,我希望它与Data.List
和Data.List.NonEmpty
兼容。 练习包含一个函数foo :: [Foo] -> Foo
并且测试套件有一些
data Case = Case
{ description :: String
, input :: [Foo]
, expected :: Foo
}
cases :: [Case]
cases =
[ Case { description = "blah blah"
, input = [Foo 1, Foo 2, Foo 3]
, expected = Foo 1
}
, ...
]
为了使用OverloadedLists
使测试套件具有多态性,我尝试了
{-# LANGUAGE OverloadedLists #-}
...
data Case list = Case
{ description :: String
, input :: list Foo
, expected :: Foo
}
cases =
[ Case { description = "blah blah"
, input = [Foo 1, Foo 2, Foo 3]
, expected = Foo 1
}
, ...
]
但这给了我错误
• Couldn't match expected type ‘GHC.Exts.Item (list0 Foo)’
with actual type ‘Foo’
The type variable ‘list0’ is ambiguous
...
|
50 | , input = [Foo 1, Foo 2, Foo 3]
| ^^^^^
我想将IsList list
约束移动到Case
数据类型,就像这样
{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedLists #-}
...
data Case list where
Case :: IsList list => String -> list Foo -> Foo -> Case list
cases =
[ Case "blah blah" [Foo 1, Foo 2, Foo 3] (Foo 1), ... ]
但这给了我错误
• Expected kind ‘* -> *’, but ‘list’ has kind ‘*’
• In the type ‘list Foo’
In the definition of data constructor ‘Case’
In the data declaration for ‘Case’
|
24 | Case :: IsList list => String -> list Foo -> Foo -> Case list
| ^^^^^^^^
我不确定这里最简单的方法是什么。 任何提示?
这不起作用的原因是因为List (l Foo) => l
的Item
类型本身不是Foo
。 该扩展对其进行了抽象,因此它期望您的列表文字的元素为Item (l Foo)
类型。
但是,您可以添加一个类型约束,表明这些项目确实是Foo
类型:
{-# LANGUAGE OverloadedLists #-}
data Case list = Case
{ description :: String
, input :: list Foo
, expected :: Foo
}
cases :: (IsList (l Foo), Item (l Foo) ~ Foo) => [Case l]
cases = [
Case { description = "blah blah"
, input = [Foo 1, Foo 2, Foo 3]
, expected = Foo 1
}
]
因此,我们在这里说Item (l Foo)
应该与Foo
相同。
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