如何指定两个类型变量相同？

Question

说这样的代码：

fun :: (a -> b) -> [a] -> [b]
fun f as = someLogic as
  where someLogic :: [a] -> [b]
        someLogic xs = map f xs

编译器会抱怨最后一行，“ a”的类型f与“ a”的类型someLogic不同。

那么我该怎么做才能让编译器知道两种类型的a是同一件事？