FlexibleInstances

常規例項需要：

All instance types must be of the form (T a1 ... an)
where a1 ... an are *distinct type variables*,
and each type variable appears at most once in the instance head.

這意味著，例如，雖然你可以為 [a] 建立例項，但你無法為 [Int] 建立例項。FlexibleInstances 放鬆了：

class C a where

-- works out of the box
instance C [a] where

-- requires FlexibleInstances
instance C [Int] where