关于haskell：有没有办法让GHC提供类型类的类型类限制？

Is there a way to make GHC provide the type class constraints of typed holes?

当前行为

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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也告诉我类型化的hole有Show类型的类约束，那就太好了。

误码率

GHC版本7.8.1

相关讨论

多亏了@dominiquedevriese的GHC票，现在这张卡已在GHC 8.0中修复。

由于扩展类型默认，这在GHCI中并不明显。举个例子，

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> 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()。这显然是所需的行为，尽管有一个参数需要说明扩展默认不应应用于孔(它们的常见用法是让编译器告诉您推断的类型)。

但是，如果您使用ghc编译或禁用ghci中的扩展默认规则(通过:set -XNoExtendedDefaultRules)，我们会看到改进的结果：

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<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)

号

不，目前是不可能的，但可以根据推测加入温室气体控制。