| 6.10 |
Module types (module specifications) |
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Module types are the module-level equivalent of type expressions: they
specify the general shape and type properties of modules.
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module-type |
::= |
modtype-path |
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sig { specification [;;] } end |
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functor ( module-name : module-type ) -> module-type |
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module-type with mod-constraint { and mod-constraint } |
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( module-type ) |
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specification |
::= |
val value-name : typexpr |
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external value-name : typexpr = external-declaration |
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type-definition |
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exception constr-decl |
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class-specification |
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classtype-definition |
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module module-name : module-type |
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module module-name { ( module-name : module-type ) }
: module-type |
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module type modtype-name |
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module type modtype-name = module-type |
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open module-path |
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include module-type |
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mod-constraint |
::= |
type [type-parameters] typeconstr = typexpr |
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module module-path = extended-module-path |
The expression modtype-path is equivalent to the module type bound
to the name modtype-path.
The expression ( module-type ) denotes the same type as
module-type.
Signatures are type specifications for structures. Signatures
sig ... end are collections of type specifications for value
names, type names, exceptions, module names and module type names. A
structure will match a signature if the structure provides definitions
(implementations) for all the names specified in the signature (and
possibly more), and these definitions meet the type requirements given
in the signature.
For compatibility with Caml Light, an optional ;; is allowed after each
specification in a signature. The ;; has no semantic meaning.
A specification of a value component in a signature is written
val value-name : typexpr, where value-name is the name of the
value and typexpr its expected type.
The form external value-name : typexpr = external-declaration
is similar, except that it requires in addition the name to be
implemented as the external function specified in external-declaration
(see chapter 18).
A specification of one or several type components in a signature is
written type typedef { and typedef } and consists of a sequence
of mutually recursive definitions of type names.
Each type definition in the signature specifies an optional type
equation = typexp and an optional type representation
= constr-decl ... or = { label-decl ... }.
The implementation of the type name in a matching structure must
be compatible with the type expression specified in the equation (if
given), and have the specified representation (if given). Conversely,
users of that signature will be able to rely on the type equation
or type representation, if given. More precisely, we have the
following four situations:
-
Abstract type: no equation, no representation.
-
Names that are defined as abstract types in a signature can be
implemented in a matching structure by any kind of type definition
(provided it has the same number of type parameters). The exact
implementation of the type will be hidden to the users of the
structure. In particular, if the type is implemented as a variant type
or record type, the associated constructors and fields will not be
accessible to the users; if the type is implemented as an
abbreviation, the type equality between the type name and the
right-hand side of the abbreviation will be hidden from the users of the
structure. Users of the structure consider that type as incompatible
with any other type: a fresh type has been generated.
- Type abbreviation: an equation = typexp, no representation.
-
The type name must be implemented by a type compatible with typexp.
All users of the structure know that the type name is
compatible with typexp.
- New variant type or record type: no equation, a representation.
-
The type name must be implemented by a variant type or record type
with exactly the constructors or fields specified. All users of the
structure have access to the constructors or fields, and can use them
to create or inspect values of that type. However, users of the
structure consider that type as incompatible with any other type: a
fresh type has been generated.
- Re-exported variant type or record type: an equation,
a representation.
-
This case combines the previous two: the representation of the type is
made visible to all users, and no fresh type is generated.
The specification exception constr-decl in a signature requires the
matching structure to provide an exception with the name and arguments
specified in the definition, and makes the exception available to all
users of the structure.
A specification of one or several classes in a signature is written
class class-spec { and class-spec } and consists of a sequence
of mutually recursive definitions of class names.
Class specifications are described more precisely in
section 6.9.4.
| Class type specifications |
|
A specification of one or several classe types in a signature is
written class type classtype-def { and classtype-def } and
consists of a sequence of mutually recursive definitions of class type
names. Class type specifications are described more precisely in
section 6.9.5.
A specification of a module component in a signature is written
module module-name : module-type, where module-name is the
name of the module component and module-type its expected type.
Modules can be nested arbitrarily; in particular, functors can appear
as components of structures and functor types as components of
signatures.
For specifying a module component that is a functor, one may write
module module-name ( name1 : module-type1 )
... ( namen : module-typen )
: module-type
instead of
module module-name :
functor ( name1 : module-type1 ) -> ...
-> module-type
| Module type specifications |
|
A module type component of a signature can be specified either as a
manifest module type or as an abstract module type.
An abstract module type specification
module type modtype-name allows the name modtype-name to be
implemented by any module type in a matching signature, but hides the
implementation of the module type to all users of the signature.
A manifest module type specification
module type modtype-name = module-type
requires the name modtype-name to be implemented by the module type
module-type in a matching signature, but makes the equality between
modtype-name and module-type apparent to all users of the signature.
The expression open module-path in a signature does not specify
any components. It simply affects the parsing of the following items
of the signature, allowing components of the module denoted by
module-path to be referred to by their simple names name instead of
path accesses module-path . name. The scope of the open
stops at the end of the signature expression.
The expression include module-type in a signature performs textual
inclusion of the components of the signature denoted by module-type.
It behaves as if the components of the included signature were copied
at the location of the include. The module-type argument must
refer to a module type that is a signature, not a functor type.
The module type expression
functor ( module-name : module-type1 ) -> module-type2
is the type of functors (functions from modules to modules) that take
as argument a module of type module-type1 and return as result a
module of type module-type2. The module type module-type2 can
use the name module-name to refer to type components of the actual
argument of the functor. No restrictions are placed on the type of the
functor argument; in particular, a functor may take another functor as
argument (``higher-order'' functor).
Assuming module-type denotes a signature, the expression
module-type with mod-constraint { and mod-constraint } denotes
the same signature where type equations have been added to some of the
type specifications, as described by the constraints following the
with keyword. The constraint type [type-parameters] typeconstr
= typexp adds the type equation = typexp to the specification
of the type component named typeconstr of the constrained signature.
The constraint module module-path = extended-module-path adds
type equations to all type components of the sub-structure denoted by
module-path, making them equivalent to the corresponding type
components of the structure denoted by extended-module-path.
For instance, if the module type name S is bound to the signature
sig type t module M: (sig type u end) end
then S with type t=int denotes the signature
sig type t=int module M: (sig type u end) end
and S with module M = N denotes the signature
sig type t module M: (sig type u=N.u end) end
A functor taking two arguments of type S that share their t component
is written
functor (A: S) (B: S with type t = A.t) ...
Constraints are added left to right. After each constraint has been
applied, the resulting signature must be a subtype of the signature
before the constraint was applied. Thus, the with operator can
only add information on the type components of a signature, but never
remove information.