当前行为
Prelude> show _
<interactive>:7:6:
Found hole ‘_’ with type: a0
Where: ‘a0’ is an ambiguous type variable
Relevant bindings include it :: String (bound at <interactive>:7:1)
In the first argument of ‘show’, namely ‘_’
In the expression: show _
In an equation for ‘it’: it = show _
期望的行为
如果 GHC 也能告诉我类型化的洞具有 Show
类型类约束,那就太好了。
其他
GHC 版本 7.8.1
最佳答案
感谢 @DominiqueDevriese 的 GHC ticket,该问题现已在 GHC 8.0 中修复。 .
由于 extended type defaulting ,这在 GHCi 中并不是立即显而易见的。以你的例子来说,
> show _
<interactive>:7:6: error:
• Found hole: _h :: ()
Or perhaps ‘_h’ is mis-spelled, or not in scope
• In the first argument of ‘show’, namely ‘_h’
In the expression: show _h
In an equation for ‘it’: it = show _h
• Relevant bindings include
it :: String (bound at <interactive>:7:1)
孔的类型默认为()
。这显然是desired behavior ,尽管有一个论点认为扩展默认值不应应用于漏洞(因为它们的常见用途是让编译器告诉您推断的类型)。
尽管如此,如果您使用 GHC 进行编译或在 GHCi 中禁用扩展默认规则(通过 :set -XNoExtendedDefaultRules
),我们会看到改进的结果:
<interactive>:3:1: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘show’
prevents the constraint ‘(Show a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Show Ordering -- Defined in ‘GHC.Show’
instance Show Integer -- Defined in ‘GHC.Show’
instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
...plus 22 others
...plus 11 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: show _
In an equation for ‘it’: it = show _
<interactive>:3:6: error:
• Found hole: _ :: a0
Where: ‘a0’ is an ambiguous type variable
• In the first argument of ‘show’, namely ‘_’
In the expression: show _
In an equation for ‘it’: it = show _
• Relevant bindings include
it :: String (bound at <interactive>:3:1)
关于haskell - 有没有办法让 GHC 提供类型孔的类型类约束?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23028124/