Using type variable from declaration in function

Question

I'm quite new to Haskell and I'm trying to use a type variable from the function declaration in the creation (not quite sure what to call this in Haskell) of a record type.
As always, code explains my problem a lot better:

data S a = S {x::a}

f :: a -> S a
f n = (S a){x=n}

GHC says that a is not in scope on the last line. How could this be accomplished?

Answer 1

使用fn = S {x=n} ，不需要类型变量

Answer 2

如果确实需要声明中的类型变量，请使用作用域类型变量扩展名。