Every value has a read-only bit. If the read-only bit is set, it means that the value is not mutable. A value's read-only bit cannot be changed after the value -is constructed. Some basic types are inherently immutable: the read-only bit is -always set for a value that belongs to an inherently immutable basic type. All -simple types are inherently immutable. Other basic types are selectively -immutable: a type is selectively immutable if it is possible to construct both -values of the type that have the read-only bit set and values that do not have -the read-only bit set. All structured types are selectively immutable. Finally, -some basic types are inherently mutable: the read-only bit is never set for a -value belonging to an inherently mutable basic type. +is constructed. Immutability is deep: the values contained in a structured value +that has its read-only bit set will all have their read-only bits set. Some +basic types are inherently immutable: the read-only bit is always set for a +value that belongs to an inherently immutable basic type. All simple types are +inherently immutable. Other basic types are selectively immutable: a type is +selectively immutable if it is possible to construct both values of the type +that have the read-only bit set and values that do not have the read-only bit +set. All structured types are selectively immutable. Finally, some basic types +are inherently mutable: the read-only bit is never set for a value belonging to +an inherently mutable basic type.
A shape is divided into two aspects: the primary aspect and the read-only aspect. A value's read-only bit is a part of the read-only aspect of the value's shape. The read-only bit of values contained in a structured value is part of the read-only aspect of those values -and of the read-only aspectof the structured value. Everything about a shape +and of the read-only aspect of the structured value. Everything about a shape except the read-only bits constitutes the primary aspect of the shape.
@@ -620,6 +622,13 @@
+Type descriptors other than readonly describes types using only the
+primary aspect of shape: whether a value belongs to the type is not affected by
+the read-only aspect of the value's shape. The readonly type uses
+only the read-only aspect: whether a value belongs to the readonly
+type depends only on the read-only aspect of the value's shape.
+
In addition to describing a type, a type descriptor may also include information used to construct a value of the type, as well as metadata. Whereas the type described by a type descriptor is known at compile time, this additional @@ -709,6 +718,9 @@
+All simple basic types are inherently immutable. +
simple-type-descriptor := @@ -1017,7 +1029,7 @@XML
The name of an element is represented by a string. The attributes of an element -are represented by a value of type
map<string>. The content +are represented by a value of typemap<string>. The children of an element is represented by a value of typexml.@@ -1025,15 +1037,15 @@
XML
identity. Other xml values do not.-An xml value is immutable if and only if all its constituent singletons are -immutable. A text singleton is inherently immutable, since it does not have -storage identity. The other singleton types are usually mutable, but can be -constructed as immutable in two ways. First, an xml value constructed by a -compile-time constant expression is immutable. Second, an immutable, deep copy -can be made of a structure by using the ImmutableClone abstract operation. Element -immutability is deep: the content of an immutable XML element is always -immutable. +The
xml:Texttype is inherently immutable. This implies that both +text singletons and empty xml values always have their read-only bits set. The +xml:Element,xml:ProcessingInstructionand +xml:Commenttypes are selectively immutable. The read-only bit of a +xml value with length greater than one is set if and only if the read-only bit +of all its constituent items is set. Immutability of xml values is deep: if the +read-only bit anxml:Elementis set, then the read-only bits of the +mapping representing its attributes and of the xml value representing its +children are also set.Note that although the mutable constituents of mutable xml value can be mutated, @@ -1144,38 +1156,32 @@
XML namespaces
Structured values
-There are three basic types of structured value: list, mapping and table. -
--A structured value is either mutable or immutable; whether it is mutable or -immutable is fixed when the value is constructed and cannot be changed -thereafter. Immutability is deep: an immutable structured value cannot refer to -a mutable structured value. The error basic type is inherently immutable: a -value of the error basic type is always immutable. Structured values of other -basic types are usually mutable, but can be constructed as immutable in two -ways. First, a structural value constructed by a compile-time constant -expression is always immutable. Second, an immutable, deep copy can be made of a -structure by using the ImmutableClone abstract -operation. +Structured values are containers for other values, which are called their +members. The shape of the members of a structured value contributes to the shape +of the structured value. A structured type descriptor describe the shape of the +structured value in terms of the shapes of its members. There are three basic +types of structured value: list, mapping and table.
-Values of the container basic types are containers for other values, which are -called their members. The shape of the members of a container value contribute -to the shape of the container. Mutating a member of a container can thus cause -the shape of the container to change. +Structured values are usually mutable. Mutating a structured value changes which +values it contains. Structured values can also be constructed as immutable. +Immutability is deep: immutable structured values cannot contain mutable +structured values; if the read-only bit of a structured value is set, then the +read-only bit of each of its members is set.
-A type descriptor for a container basic type describe the shape of the container -in terms of the shapes of its members. A container has an inherent type, which -is a type descriptor which is part of the container's runtime value. At runtime, -the container prevents any mutation that might lead to the container having a -shape that is not a member of its inherent type. Thus a container value belongs -to a type if and only if that its inherent type is a subtype of that type. +Mutating a member of a structured value can cause the shape of the structured +value to change. A structured value has an inherent type, which is a type +descriptor which is part of the structured values's runtime value. At runtime, +the structured value prevents any mutation that might lead to the structured +value having a shape that is not a member of its inherent type. Thus a +structured value belongs to a type if and only if that its inherent type is a +subtype of that type.
-The inherent type of an immutable container is a singleton type with the -container's shape as its single member. Thus, an immutable container value -belongs to a type if and only if the type contains the shape of the value. +The inherent type of an immutable structured value is a singleton type with the +structured value's shape as its single member. Thus, an immutable structured +value belongs to a type if and only if the type contains the shape of the value.
Each member of a container has a key that uniquely identifies it within the @@ -1346,7 +1352,6 @@
Mappings
sequence is implementation-dependent, but implementations are encouraged to preserve and use the order in which the fields were added. -The type of mapping values can be described by two kinds of type descriptors.
@@ -1577,9 +1582,11 @@Error
inferred-error-type-param :=*
-An error value belongs to the error basic type, which is used only for -representing errors. The error type is inherently immutable. An error value -contains the following information: +An error value is used to represent an error. Errors are a distinct basic type: +a value belongs to the error basic type if and only if it is an error. This +means that errors are clearly distinguished from other values. Many constructs +in Ballerina give special treatment to error values. The error type is +inherently immutable. An error value contains the following information:
ImmutableClone(v) is defined for any value v that belongs to the type
anydata|readonly. It performs a deep copy of v similar to Clone(v),
-except that newly constructed values are constructed as immutable. Any immutable
-value is not copied.
+except that newly constructed values will be constructed as immutable and so
+have their read-only bit set. Any immutable value is not copied. So the result
+of Immutable always has its read-only bit set.
Like Clone, ImmutableClone must preserve graph structure, including cycles. @@ -3267,7 +3275,7 @@
In most cases, the precise type and the broad type of an expression are the @@ -3340,6 +3348,10 @@
( const-expr )
+A value resulting from the evaluation of a const-expr always has
+its read-only bit set.
+
Within a const-expr, any nested expression must also be a
const-expr. A functional-constructor-expr within a
const-expr must not construct an error value.
@@ -3480,6 +3492,10 @@
+An list-constructor-expr occurring within a const-expr will construct a list value +that has its read-only bit set. +
+An mapping-constructor-expr occurring within a const-expr will construct a mapping value +that has its read-only bit set. +
+A table-constructor-expr occurring within a const-expr will construct a table +value that has its read-only bit set. +
+xml BacktickString
-An XML template expression creates an xml value as follows: +An XML template expression constructs an xml value as follows:
+An xml-template-expr occurring within a const-expr will construct an xml value +that has its read-only bit set. +
-A member-access-expr accesses a member of a container value using its key, +A member-access-expr accesses a member of a structured value using its key, or a constituent of a sequence value using its index.
-The requirements on the static type of container-expr and key-expression are as +The requirements on the static type of container-expression and key-expression are as follows:
+A functional-constructor-expr occurring within a const-expr will construct a +value with its read-only bit set. +
-The type-descriptor provides the contextually expected type for
-expression.
+The contextually expected type for expression is the intersection
+of the contextually expected type of the type-cast-expr and the
+type described by the type-descriptor.
+Note that a type-cast-expr of <readonly> can be
+used both to cause constructors withing the expression to construct
+values with the read-only bit set and to verify that the value resulting from
+the evaluation of expression has its read-only bit set.
+
-The type of the constant is the singleton type containing just the shape of the
-value named by the constant. The type of the constant determines the static type
-of a variable-reference-expr that references this constant.
+The type of the constant is the intersection of readonly and the
+singleton type containing just the shape of the value named by the constant. The
+type of the constant determines the static type of a variable-reference-expr
+that references this constant.
If type-descriptor is present, then it provides the contextually expected type