XML Path Language (XPath) 4.0 WG Review Draft

3.4.2 Item Coercion Rules

The rules in this section are used to process each item J in a supplied sequence, given a required item typeR.

1. If R is a generalized atomic type (for example, if it is an atomic type, a pure union type, or an enumeration type), and J is not an atomic item, then:

   1. J is atomized to produce a sequence of atomic items JJ.J is atomized to produce a sequence of atomic items JJ.J is atomizedatomized to produce a sequence of atomic items JJ.

   2. Each atomic item in JJ is coerced to the required type R by recursive application of the item coercion rules (the rules in this section) to produce a value V.

   3. The result is the sequence-concatenation of the V values.The result is the sequence concatenation of the V values.The result is the sequence-concatenationsequence concatenation of the V values.

   Note:

   For example, if J is an element with type annotation xs:integer, and R is the union type xs:numeric, then the effect is to atomize the element to an xs:integer, and then to coerce the resulting xs:integer to xs:numeric (which leaves the integer unchanged). This is not the same as attempting to coerce the element to each of the alternatives of the union type in turn, which would deliver an instance of xs:double.

2. Otherwise, if R is a choice item type or a pure union type (which includes the case where it is an enumeration type), then:

   1. If J matches (is an instance of) one of the alternatives in R, then J is coerced to the first alternative in R that J matches.

      Note:

      There are two situations where coercing an item to a type that it already matches does not simply return the item unchanged:

      • When the required type is a typed function type (see 3.2.8.1 Function Types), then function coercion is applied to coerce J to that function type, as described in 3.4.4 Function Coercion.

      • When the required type is a record type and the supplied value is a map, then coercion may change the entry order^DM of the entries in the map.

   2. Otherwise, the item coercion rules (the rules in this section) are applied to J recursively with R set to each of the alternatives in the choice or union item type, in order, until an alternative is found that does not result in a type error; a type error is raised only if all alternatives fail.

      The error code used in the event of failure should be the error code arising from the first unsuccessful matching attempt. (The diagnostic information associated with the error may also describe how further attempts failed.)

   Note:

   Suppose the required type is (xs:integer | element(e))* and the supplied value is the sequence (<e>22</e>, 23, <f>24</f>). Item coercion is applied independently to each of the three items in this sequence. The first item matches one of the alternatives, namely element(e), so it is returned unchanged as an element node. The second item (the integer 23) also matches one of the alternatives, and is returned unchanged as an integer. The third item does not match any of the alternatives, so coercion is attempted to each one in turn. Coercion to type xs:integer succeeds (by virtue of atomization and untyped atomic conversion), so the final result is the sequence (<e>22</e>, 23, 24)

   Note:

   Suppose the required type is enum("red", "green", "blue") and the supplied value is "green". The enumeration type is defined as a choice item type whose alternatives are singleton enumerations, so the rules are applied first to the type enum("red") (which fails), and then to the type enum("green") (which succeeds). The strings in an enumeration type are required to be distinct so the order of checking is in this case immaterial. The supplied value will be accepted, and will be relabeled if necessary as an instance of xs:string.

   Note:

   Schema-defined union types behave in exactly the same way as choice item types.

3. If R is an atomic type and J is an atomic item, then:

   1. If J is an instance of R then it is used unchanged.

   2. If J is an instance of type xs:untypedAtomic then:

      1. If R is namespace-sensitive then a type error [err:XPTY0117] is raised.

      2. Otherwise, J is cast to type R.

4. If there is an entry (from, to) in the following table such that J is an instance of from, and to is R, then J is cast to type R.

   Implicit Casting

   from	to
   xs:decimal	xs:double
   xs:double	xs:decimal
   xs:decimal	xs:float
   xs:float	xs:decimal
   xs:float	xs:double
   xs:double	xs:float
   xs:string	xs:anyURI
   xs:anyURI	xs:string
   xs:hexBinary	xs:base64Binary
   xs:base64Binary	xs:hexBinary

   Note:

   The item type in the to column must match R exactly; however, J may belong to a subtype of the type in the from column.

   For example, an xs:NCName will be cast to type xs:anyURI, but an xs:anyURI will not be cast to type xs:NCName.

   Similarly, an xs:integer will be cast to type xs:double, but an xs:double will not be cast to type xs:integer.

5. If R is an singleton enumeration type and J is an instance of xs:untypedAtomic or xs:anyURI, then J is cast to type xs:string.

   Note:

   The effect of this rule, when taken in conjunction with the rules above regarding atomization and choice item types, is that when the required type is based on an enumeration type, for example enum("red", "green", "blue")*, the supplied value can be, in this example:

   • An empty sequence.

   • The string "red".

   • An untyped node whose string value is "red".

   • A list-valued node whose typed value contains zero or more strings (or xs:anyURI values), each of which is codepoint-equal to one of "red", "green", or "blue".

   • The xs:anyURI value "red".

   • An array with zero or more members each of which is codepoint-equal to one of "red", "green", or "blue".

   • A JNode whose ·content· property is any of the above.

6. If R is an singleton enumeration type and J is an instance of xs:untypedAtomic or xs:anyURI, then J is cast to type xs:string.

   Note:

   The effect of this rule, when taken in conjunction with the rules above regarding atomization and choice item types, is that when the required type is based on an enumeration type, for example enum("red", "green", "blue")*, the supplied value can be, in this example:

   • An empty sequence.

   • The string "red".

   • An untyped node whose string value is "red".

   • A list-valued node whose typed value contains zero or more strings (or xs:anyURI values), each of which is codepoint-equal to one of "red", "green", or "blue".

   • The xs:anyURI value "red".

   • An array with zero or more members each of which is codepoint-equal to one of "red", "green", or "blue".

   • A JNode whose ·content· property is any of the above.

7. If R is derived from some primitive atomic type P, then J is relabeled as an instance of R if it satisfies all the following conditions:

   • J is an instance of P.

   • J is not an instance of R.

   • The datum^DM of J is within the value space of R.

   Relabeling an atomic item changes the type annotation but not the datum^DM. For example, the xs:integer value 3 can be relabeled as an instance of xs:unsignedByte, because the datum is within the value space of xs:unsignedByte.

   Note:

   Relabeling is not the same as casting. For example, the xs:decimal value 10.1 can be cast to xs:integer, but it cannot be relabeled as xs:integer, because its datum not within the value space of xs:integer.

   Note:

   The effect of this rule is that if, for example, a function parameter is declared with an expected type of xs:positiveInteger, then a call that supplies the literal value 3 will succeed, whereas a call that supplies -3 will fail.

   This differs from previous versions of this specification, where both these calls would fail.

   This change allows the arguments of existing functions to be defined with a more precise type. For example, the $position argument of array:get could be defined as xs:positiveInteger rather than xs:integer.

   Note:

   If T is a union type with members xs:negativeInteger and xs:positiveInteger and the supplied value is the sequence (20, -20), then the effect of these rules is that the first item 20 is relabeled as type xs:positiveInteger and the second item -20is relabeled as type xs:negativeInteger.

   Note:

   Promotion (for example of xs:float to xs:double) occurs only when T is a primitive type. Relabeling occurs only when T is a derived type. Promotion and relabeling are therefore never combined.

   Note:

   A singleton enumeration type such as enum("green") is treated as an atomic type derived by restriction from xs:string; so if the xs:string value "green" is supplied in a context where the required type is enum("red", "green", "blue"), the value will be accepted.

8. If J is a JNode^DM and does not match R, then each item in the ·content· of J is coerced to type R by applying the coercion rules recursively.

   Note:

   For example, if $A is an array and the members of the array are maps, then $A/child::* returns a sequence of JNodes that encapsulate maps, and the average size of these maps can be obtained using the expression avg($A/child::* ! map:size(.)). The first argument of map:size does not accept a JNode directly, but it does (in effect) accept a JNode that encapsulates a map.

9. If R is an ArrayType other than array(*) and J is an array, then J is converted to a new array by converting each member to the required member type by applying the coercion rules recursively.

   Note:

   For example, if the required type is array(xs:double) and the supplied value is [ 1, 2 ], the array is converted to [ 1e0, 2e0 ].

10. If R is a MapType other than map(*) and J is a map, then J is converted to a new map as follows:

    1. Each key in the supplied map is converted to the required map key type by applying the coercion rules. If the resulting map would contain duplicate keys, a type error is raised [err:XPTY0004].

    2. The corresponding value is converted to the required map value type by applying the coercion rules recursively.

    3. The order of entries in the map remains unchanged.

    Note:

    For example, if the required type is map(xs:string, xs:double) and the supplied value is { "x": 1, "y": 2 }, the map is converted to { "x": 1e0, "y": 2e0 }.

    Note:

    Duplicate keys can occur if the value space of the target type is more restrictive than the original type. For example, an error is raised if the map { 1.2: 0, 1.2000001: 0 }, which contains two keys of type xs:decimal, is coerced to the type map(xs:float, xs:integer).

11. If R is a RecordType and J is a map, then J is converted to a new map as follows:

    1. The keys in the supplied map are unchanged.

    2. In any map entry whose key is equal to the name of one of the field declarations in R (under the rules of the atomic-equal function), the corresponding value is converted to the required type defined by that field declaration, by applying the coercion rules recursively (but with XPath 1.0 compatibility mode treated as false).

    3. The order of entries in the map is changed: entries whose keys correspond to the names of field declarations in R appear first, in the order of the corresponding field declarations, and (if the record type is extensible) other entries then follow retaining their relative order in J.

    Note:

    For example, if the required type is record(longitude as xs:double, latitude as xs:double) and the supplied value is { "latitude": 53.2, "longitude": 0 }, then the map is converted to { "longitude": 0.0e0, "latitude": 53.2e0 }.

12. If R is a TypedFunctionType and J is a function item, then function coercion is applied to J.

    Note:

    Function coercion applies even if J is already an instance of R.

    Maps and arrays are functions, so function coercion applies to them as well.

13. If, after the above conversions, the resulting item does not match the expected item type R according to the rules for SequenceType Matching, a type error is raised [err:XPTY0004].

    Note:

    Under the general rules for type errors (see 2.4.1 Kinds of Errors), a processor may report a type error during static analysis if it will necessarily occur when the expression is evaluated. For example, the function call fn:abs("beer") will necessarily fail when evaluated, because the function requires a numeric value as its argument; this may be detected and reported as a static error.