8.4 Functional record update

SML# contains functional record update expressions, whose syntax is given below.

	$\displaystyle expr$	$\displaystyle\mbox{\ \ }::=$	$\displaystyle\cdots$
		$\|$	$e x p r$ # { $l_{1}$ = $expr_{1}$ , $\cdots$ , $l_{n}$ = $expr_{n}$ }

This expression creates a new record by modifying the value of each label $l_{i}$ to $expr_{i}$ . This is an expression to create a new record; the original record $e x p r$ is not mutated. This expression has a polymorphic type according to the ML’s principle of most general typing. Below is an example using this expression.

# fun f modify x = modify # {X = x};
val f = fn : [’a#{X: ’b}, ’b. ’a -> ’b -> ’a]

The following is a useful idiom in record programming.

# fun reStructure (p as {Salary,...}) = p # {Salary = Salary * (1.0 - 0.0803)};
val reStructure = fn : [’a#{Salary: real}. ’a -> ’a]

Function reStructure takes an employee record p and reduces its Salary field by 8.03%. As seen in its typing, this function can be applied to any record as far as it contains a ，Salary field.