GHC extensions
since 7.8
The ambiguity check rejects functions that can never be called.
since 6.8.1
Enable lexical scoping of type variables explicitly introduced with forall.
since 8.0.1
Allow the use of type application syntax. (类型构造器)
本来只能：
> show (read "5"::Int)
"5"
现在可以使用这种语法：
> show (read @Int "5" )
"5"
facilitate type-level programming
