为什么 `a:: nil` 中的 `a` 代表长度为 1 的列表？
Why does `a` in `a :: nil` represent a list of length 1?
在 Ullman 的 SML 书中：
Note that the way we express "list of length 1" as a pattern is to put
square brackets around a single identifier, like [a].
Such a pattern can only match a list with a single element, and variable a
acquires that element as its value.
Another way to express "list of length 1" is with the pattern a :: nil.
Again, a acquires the lone element as its value.
在a :: nil中，a是否表示列表的头元素？为什么表示长度为1的列表？
谢谢。
a 是列表的头部，nil 是列表的尾部。 nil 是一个长度为零的列表，因此 a :: nil 的长度为 1。
在 Ullman 的 SML 书中：
Note that the way we express "list of length 1" as a pattern is to put square brackets around a single identifier, like
[a]. Such a pattern can only match a list with a single element, and variable a acquires that element as its value.Another way to express "list of length 1" is with the pattern
a :: nil. Again,aacquires the lone element as its value.
在a :: nil中，a是否表示列表的头元素？为什么表示长度为1的列表？
谢谢。
a 是列表的头部，nil 是列表的尾部。 nil 是一个长度为零的列表，因此 a :: nil 的长度为 1。