有没有办法让GHC提供types孔types的约束？
目前的行为
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也告诉我这个types的孔有Show类的types约束，那将会很好。
杂项
GHC版本7.8.1
现在，由于DominiqueDevriese的GHC票证，现在已经在GHC 8.0中修复了。
由于扩展types的违约 ，这在GHCi中并不是显而易见的。 以你为例，
> 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types默认为() 。 这显然是所期望的行为 ，尽pipe有一个论点需要指出，延期违约不应该适用于漏洞（因为他们通常使用的是让编译器告诉你推断的types）。
不过，如果使用GHC编译或在GHCi中禁用扩展默认规则（通过:set -XNoExtendedDefaultRules ），我们可以看到改进的结果：
<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)
目前没有这个可能。但是根据推测它可能被添加到GHC。
