19.4 Record expression {$\mathrm{⟨}$lab${}_{\mathrm{1}}$$\mathrm{⟩}$ = $\mathrm{⟨}$exp${}_{\mathrm{1}}$$\mathrm{⟩}$, $\mathrm{\dots }$, $\mathrm{⟨}$lab${}_n$$\mathrm{⟩}$ = $\mathrm{⟨}$exp${}_n$$\mathrm{⟩}$}

A record expression is a collection of pairs of labels $⟨$lab${}_i$$\mathrm{⟩}$ and expressions $⟨$exp${}_i$$\mathrm{⟩}$.

A record expression is evaluated as follows. First, it is checked whether all of its labels are distinct. If not, a type error occurs. Second, each expression $⟨$exp${}_i$$\mathrm{⟩}$ is evaluated to a type $⟨$ty${}_i$$\mathrm{⟩}$ and value $v{}_i$ in the order of their occurrences in the record expression. Finally, they are sorted in the order of label strings and constitutes a record type

{$\mathrm{⟨}$lab${}_{\mathrm{1}}{}^{\mathrm{\prime }}$$\mathrm{⟩}$ : $\mathrm{⟨}$ty${}_{\mathrm{1}}{}^{\mathrm{\prime }}$$\mathrm{⟩}$, $\mathrm{\dots }$, $\mathrm{⟨}$lab${}_n{}^{\mathrm{\prime }}$$\mathrm{⟩}$ : $\mathrm{⟨}$ty${}_n{}^{\mathrm{\prime }}$$\mathrm{⟩}$}

and record value

{$\mathrm{⟨}$lab${}_{\mathrm{1}}{}^{\mathrm{\prime }}$$\mathrm{⟩}$ = $v{}_{\mathrm{1}}{}^{\mathrm{\prime }}$, $\mathrm{\dots }$, $\mathrm{⟨}$lab${}_n{}^{\mathrm{\prime }}$$\mathrm{⟩}$ = $v{}_n{}^{\mathrm{\prime }}$}

as the type and value of the record expression.

Therefore, for example, when

val x = {SML = (print "SML#";"SML#"), IS = (print " is ";" is "), SHARP = (print " sharp!\n";" sharp!\n")}

is evaluated, it prints "SML# is sharp!" and then x is bound to the type and value like the following

val x = {IS = " is ", SHARP = " sharp!\n", SML = "SML#"} : {IS: string, SHARP: string, SML: string}