Polymorphic Record Update

SML# provides the following stntax

 <expr>  ::= <expr> # { <labeledFields> }
 <labeledFields> ::= <label> = <expr> | <label> = <expr>, <labeledFields>

for functional polymorphic record update. <expr> # { <labeledFields> } creates a new record from <expr> by changing the field values specified in <labeledFields>. It is required that all the specified fields in <labeldFields> must be present in <expr>.