Allow provider-specific data types

optimade.rst

@@ -220,6 +220,7 @@ representation in all contexts. They are as follows:
   A list can be empty, i.e., contain no items.
 - **dictionary**: an associative array of **keys** and **values**, where **keys** are pre-determined strings, i.e., for the same entry property, the **keys** remain the same among different entries whereas the **values** change.
   The **values** of a dictionary can be any basic type, list, dictionary, or unknown.
+- Database-provider-specific or definition-provider-specific data type.
 
 An entry property value that is not present in the database is **unknown**.
 This is equivalently expressed by the statement that the value of that entry property is :val:`null`.
@@ -627,6 +628,7 @@ In the JSON response format, property types translate as follows:
 - **timestamp** uses a string representation of date and time as defined in `RFC 3339 Internet Date/Time Format <https://tools.ietf.org/html/rfc3339#section-5.6>`__.
 - **dictionary** is represented by the JSON object type.
 - **unknown** properties are represented by either omitting the property or by a JSON :field-val:`null` value.
+- database-provider-specific or definition-provider-specific data types use string representations.
 
 Every response SHOULD contain the following fields, and MUST contain at least :field:`meta`:
 
@@ -1816,6 +1818,7 @@ The following tokens are used in the filter query component:
   (Note that at the end of the string value above the four final backslashes represent the two terminal backslashes in the value, and the final double quote is a terminator, it is not escaped.)
 
   String value tokens are also used to represent **timestamps** in form of the `RFC 3339 Internet Date/Time Format <https://tools.ietf.org/html/rfc3339#section-5.6>`__.
+  String value tokens as well are used to represent database-provider-specific or definition-provider-specific data types.
 
 - **Numeric values** are represented as decimal integers or in scientific notation, using the usual programming language conventions.
   A regular expression giving the number syntax is given below as a `POSIX Extended Regular Expression (ERE) <https://en.wikipedia.org/w/index.php?title=Regular_expression&oldid=786659796#Standards>`__ or as a `Perl-Compatible Regular Expression (PCRE) <http://www.pcre.org>`__:
@@ -2070,6 +2073,10 @@ As the filter language syntax does not define a lexical token for timestamps, va
 In a comparison with a timestamp property, a string token represents a timestamp value that would result from parsing the string according to RFC 3339 Internet Date/Time Format.
 Interpretation failures MUST be reported with error :http-error:`400 Bad Request`.
 
+Database and definition providers MAY introduce custom types, representing them with string lexical tokens both in filters and responses.
+It is up to the providers to decide which comparison operators to support and how the comparisons should be performed.
+For example, a provider that introduces a set-valued property :property:`_exmpl_set`, may decide to override the :val:`CONTAINS` operator so that :filter:`identifier CONTAINS set` is true when :val:`set` is a subset of a property.
+
 Optional filter features
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -2158,7 +2165,7 @@ A Property Definition MUST be composed according to the combination of the requi
 
 - :field:`x-optimade-type`: String.
   Specifies the OPTIMADE data type for this level of the defined property.
-  MUST be one of :val:`"string"`, :val:`"integer"`, :val:`"float"`, :val:`"boolean"`, :val:`"timestamp"`, :val:`"list"`, or :val:`"dictionary"`.
+  MUST be one of :val:`"string"`, :val:`"integer"`, :val:`"float"`, :val:`"boolean"`, :val:`"timestamp"`, :val:`"list"`, :val:`"dictionary"` or a string naming database-provider-specific or definition-provider-specific data type starting with provider-specific prefix.
 
 - :field:`x-optimade-unit`: String.
   A (compound) symbol for the physical unit in which the value of the defined property is given or one of the strings :val:`dimensionless` or :val:`inapplicable`.