[WIP] Add support for reading and writing variables

[WIP] Command line option, move later?

This adds a reader flag and serdi option for extending a syntax with support
for SPARQL-like variables, for storing things like patterns or simple queries.

2 files changed, 23 insertions, 4 deletions

diff --git a/include/serd/node.h b/include/serd/node.h
index b308a507..1faba160 100644
--- a/include/serd/node.h
+++ b/include/serd/node.h
@@ -38,9 +38,14 @@ typedef struct SerdNodeImpl SerdNode;
 /**
    Type of a node.
 
-   An abstract RDF node can be either a resource or a literal.  This type is
-   more precise to preserve syntactic differences and support additional
-   features.
+   Note that this set of types is both more precise than, and extended from,
+   the possible types of an abstract RDF node.  Not all types can occur in all
+   contexts, for example, a Turtle document can't contain a variable node.
+
+   The string value of a node never contains quoting or other type indicators.
+   For example, the blank node `_:id3` and the plain literal `"id3"` from a
+   Turtle document would both have the same string, "id3", returned by
+   #serd_node_string.
 */
 typedef enum {
   /**
@@ -82,6 +87,19 @@ typedef enum {
      Turtle](http://www.w3.org/TR/turtle/#grammar-production-BLANK_NODE_LABEL)
   */
   SERD_BLANK = 4,
+
+  /**
+     A variable node.
+
+     A variable's identity is, like blank nodes, local to its context.
+     Variables are typically used in interfaces for querying, rather than
+     present in data (there is no concept of a variable in RDF or its standard
+     syntaxes).
+
+     @see [SPARQL 1.1 Query
+     Language](https://www.w3.org/TR/sparql11-query/#rVar)
+  */
+  SERD_VARIABLE = 5,
 } SerdNodeType;
 
 /// Node flags, which ORed together make a #SerdNodeFlags
diff --git a/include/serd/reader.h b/include/serd/reader.h
index 4c669342..040f3398 100644
--- a/include/serd/reader.h
+++ b/include/serd/reader.h
@@ -29,7 +29,8 @@ typedef struct SerdReaderImpl SerdReader;
 
 /// Reader options
 typedef enum {
-  SERD_READ_LAX = 1U << 0U, ///< Tolerate invalid input where possible
+  SERD_READ_LAX       = 1U << 0U, ///< Tolerate invalid input where possible
+  SERD_READ_VARIABLES = 1U << 1U, ///< Support variable nodes
 } SerdReaderFlag;
 
 /// Bitwise OR of SerdReaderFlag values