SML# Document Version 3.7.1

24 Module language declarations and interface 24.2 Structure expressions and their evaluation :

\,\langle

\rangle\,

24.4 Module language interface

24.3 Signature expression : $\,\langle$ sigexp $\rangle\,$

The syntax of signature expression is given below.

$\,\langle$ sigexp $\rangle\,$	::=	sig $\,\langle$ spec $\rangle\,$ end
	$\|$	$\,\langle$ sigid $\rangle\,$
	$\|$	$\,\langle$ sigexp $\rangle\,$ where type $\,\langle$ tyvarSeq $\rangle\,$ $\,\langle$ longTycon $\rangle\,$ = $\,\langle$ ty $\rangle\,$

•

Basic signature expression (sig $\,\langle$ spec $\rangle\,$ end)

It is a list of declaration specification
•

Signature name ( $\,\langle$ sigid $\rangle\,$ )

It is a name bound to a signature expression in a top-level signature declaration.
•

Signature name with type definitions

It represent the signature expression obtained from the signature expression bound to $\,\langle$ sigin $\rangle\,$ by replacing $\,\langle$ tyvarSeq $\rangle\,$ $\,\langle$ longTycon $\rangle\,$ with the specified $\,\langle$ ty $\rangle\,$ .

The syntax of declaration specification $\,\langle$ spec $\rangle\,$ is given below.

$\,\langle$ spec $\rangle\,$	::=	val $\,\langle$ valdesc $\rangle\,$
	$\|$	type $\,\langle$ typdesc $\rangle\,$
	$\|$	eqtype $\,\langle$ typdesc $\rangle\,$
	$\|$	datatype $\,\langle$ datdesc $\rangle\,$
	$\|$	datatype $\,\langle$ tycon $\rangle\,$ = datatype $\,\langle$ longTycon $\rangle\,$
	$\|$	exception $\,\langle$ exdesc $\rangle\,$
	$\|$	structure $\,\langle$ strdesc $\rangle\,$
	$\|$	include $\,\langle$ sigexp $\rangle\,$
	$\|$	$\,\langle$ spec $\rangle\,$ sharing type $\,\langle$ longTycon ${}_{1}$ $\rangle\,$ = $\cdots$ = $\,\langle$ longTycon ${}_{n}$ $\rangle\,$
	$\|$	$\,\langle$ spec $\rangle\,$ $($ ; $)?$ $\,\langle$ spec $\rangle\,$
	$\|$
$\,\langle$ valdesc $\rangle\,$	::=	$\,\langle$ vid $\rangle\,$ : $\,\langle$ ty $\rangle\,$ $($ and $\,\langle$ valdesc $\rangle\,$ $)?$
$\,\langle$ typdesc $\rangle\,$	::=	$\,\langle$ tyvarSeq $\rangle\,$ $\,\langle$ tycon $\rangle\,$ $($ and $\,\langle$ typdesc $\rangle\,$ $)?$
$\,\langle$ datdesc $\rangle\,$	::=	$\,\langle$ tyvarSeq $\rangle\,$ $\,\langle$ tycon $\rangle\,$ = $\,\langle$ condesc $\rangle\,$ $($ and $\,\langle$ datdesc $\rangle\,$ $)?$
$\,\langle$ condesc $\rangle\,$	::=	$\,\langle$ vid $\rangle\,$ $($ of $\,\langle$ ty $\rangle\,$ $)?$ $($ \| $\,\langle$ condesc $\rangle\,$ $)?$
$\,\langle$ exdesc $\rangle\,$	::=	$\,\langle$ vid $\rangle\,$ $($ of $\,\langle$ ty $\rangle\,$ $)?$ $($ and $\,\langle$ exdesc $\rangle\,$ $)?$
$\,\langle$ strdesc $\rangle\,$	::=	$\,\langle$ strid $\rangle\,$ : $\,\langle$ sigexp $\rangle\,$ $($ and $\,\langle$ strdesc $\rangle\,$ $)?$

The meaning of each of components is as follows.

•

val $\,\langle$ valdesc $\rangle\,$ . This specifies the type of each variable bound in a val declaration.
•

type $\,\langle$ typdesc $\rangle\,$ . This specifies the existence of a type declaration or datatype declaration in a structure expression.

When the signature is used as a transparent signature constraint, then this specification makes the type function available. If the matching declaration is a datatype declaration, then this specification makes the type constructor available, but data constructors are not bound.

When the signature is used as an opaque signature constraint, then this specification changes a type function or a datatype definition to an abstract type constructor.
•

eqtype $\,\langle$ typdesc $\rangle\,$

This is the same as type $\,\langle$ typdesc $\rangle\,$ with the additional constraint that the type declared in a structure is restricted to a type that admit equality.
•

datatype $\,\langle$ datdesc $\rangle\,$

This specifies that a datatype declaration exists, and makes the datatype declaration available.
•

datatype $\,\langle$ tycon $\rangle\,$ = datatype $\,\langle$ longTycon $\rangle\,$

It specifies that the type constructor bound to the type constructor name $\,\langle$ tycon $\rangle\,$ is the same as that bound to the long type constructor name $\,\langle$ longTycon $\rangle\,$ .
•

exception $\,\langle$ exdesc $\rangle\,$

This specifies that an exception declaration exists, and makes the exception declaration available.
•

structure $\,\langle$ strdesc $\rangle\,$

It specifies that the structure contain a structure declaration of the specified signature.
•

include

It expands to the contents (a list of $\,\langle$ spec $\rangle\,$ ) of the signature expression bound to the signature name.
•

sharing type

This specifies that the specified set of long type constructor names are bound to the same type constructor.

24.3 Signature expression : ⟨sigexp⟩

24.3 Signature expression : $\,\langle$ sigexp $\rangle\,$