在第三章，你第一次接触了记录类型，我并没有讨论如何更新它们的字段。这是因为，默认情况下记录类型是不可更改的。F#提供了特殊的语法，以允许更新记录类型里的字段，在记录类型的字段前面使用关键字mutable。我必须强调，这一操作改变的内容是记录的字段，而非记录本身。

#light
type Couple = { her : string ; mutable him : string }

let theCouple = { her = "Elizabeth Taylor " ; him = "Nicky Hilton" }

let print o = printf "%A\r\n" o

let changeCouple() =
     print theCouple;
     theCouple.him <- "Michael Wilding";
     print theCouple;
     theCouple.him <- "Michael Todd";
     print theCouple;
     theCouple.him <- "Eddie Fisher";
     print theCouple;、

theCouple.him <- "Richard Burton";
     print theCouple;
     theCouple.him <- "Richard Burton";
     print theCouple;
     theCouple.him <- "John Warner";
     print theCouple;
     theCouple.him <- "Larry Fortensky";
     print theCouple
changeCouple()
执行结果:
{her = "Elizabeth Taylor "; him = "Nicky Hilton"}
{her = "Elizabeth Taylor "; him = "Michael Wilding"}
{her = "Elizabeth Taylor "; him = "Michael Todd"}
{her = "Elizabeth Taylor "; him = "Eddie Fisher"}
{her = "Elizabeth Taylor "; him = "Richard Burton"}
{her = "Elizabeth Taylor "; him = "Richard Burton"}
{her = "Elizabeth Taylor "; him = "John Warner"}
{her = "Elizabeth Taylor "; him = "Larry Fortensky"}

这个例子显示一个mutable记录的动作。一个类型,couple,定义其字段him是一个mutable但字段her不是。接下来，实例化一个couple，然后改变him的值多次，每次都输出结果。请注意，mutable关键字并没有应用于每个字段，因此，任何尝试更新一个非mutable的字段的动作都会导致一个编译错误，如下所示。

#light
theCouple.her <- "Sybil Williams";
print_any theCouple
如果你编译这段代码，你将得到以下错误消息:
prog.fs(2,4): error: FS0005: This field is not mutable