The basic operation on records is to select a field value by specifying a field label. For this, the following primitive function is defined for any label $l$.

#$l$

This function takes a record containing a field labeled with $l$, and returns the value of that label. According to the ML principle, this function should be applicable to any record containing label $l$. For this primitive, SML# infers the following polymorphic typing.

# #X;
val it = fn : [’a#{X: ’b}, ’b. ’a -> ’b]

The notation ’a#{X:’b} is a type variable ’a which represents an arbitrary record type that contains a field labeled X of type ’b. The inferred type is a most general type of #X. Below show some examples.

# #X {X=1.1, Y=2.2};
val it = 1.1 : real
# #X {X = 1, Y = 2, Z = 3};
val it = 1 : int

Functions using these operations are polymorphic in record structures, as seen in the following example.

# fun f x = (#X x, #Y x);
val f = fn : [’a#{X: ’b, Y: ’c}, ’b, ’c. ’a -> ’b * ’c]

The type of f indicates that this is a function that takes any record containing X:’b and Y:’c fields and returns a pair of type ’b * ’c. This is a most general polymorphic type of this function. So this function can be freely combined with other expressions as far the combination is type consistent.