Add RDF.Datatype.cast/1

CHANGELOG.md

@@ -16,6 +16,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
   literals in Turtle encoder
 - `RDF.Numeric` with a list of all numeric datatypes and shared functions for
   all numeric literals, eg. arithmetic functions
+- `RDF.Datatype.cast/1` for casting between `RDF.Literal`s  as specified in the 
+  XSD spec 
 - the logical operators and the Effective Boolean Value (EBV) coercion algorithm 
   from the XPath and SPARQL specs on `RDF.Boolean`
 - `RDF.DateTime.now/0`

lib/rdf/datatype.ex

@@ -35,7 +35,7 @@ defmodule RDF.Datatype do
   @doc """
   Produces the canonical form of a `RDF.Literal`.
   """
-  @callback canonical(literal :: RDF.Literal.t) :: RDF.Literal.t
+  @callback canonical(RDF.Literal.t) :: RDF.Literal.t
 
   @doc """
   Converts a value into a proper native value.
@@ -51,6 +51,16 @@ defmodule RDF.Datatype do
   """
   @callback convert(any, keyword) :: any
 
+
+  @doc """
+  Casts a literal of another datatype into a literal of the datatype the function is implemented on.
+
+  If the given literal is invalid or can not be converted into this datatype
+  `nil` is returned.
+  """
+  @callback cast(RDF.Literal.t) :: RDF.Literal.t
+
+
   @doc """
   Determines if the value of a `RDF.Literal` is a member of lexical value space of its datatype.
   """