Change RDF.IRI.absolute/2 to return nil if the base is not absolute

CHANGELOG.md

@@ -29,6 +29,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 - Elixir 1.4 is no longer supported
 - `RDF.String.new/2` and `RDF.String.new!/2` produce a `rdf:langString` when 
   given a language tag
+- `RDF.IRI.absolute/2` returns `nil` if the given base is not absolute, instead 
+  of failing with a `FunctionClauseError`
 
 
 ### Fixed

lib/rdf/iri.ex

@@ -126,12 +126,14 @@ defmodule RDF.IRI do
 
   Characters additionally allowed in IRI references are treated in the same way that unreserved
   characters are treated in URI references, per [section 6.5 of RFC3987](http://tools.ietf.org/html/rfc3987#section-6.5)
+
+  If the given is not an absolute IRI `nil` is returned.
   """
   def absolute(iri, base) do
-    if absolute?(iri) do
-      new(iri)
-    else
-      merge(base, iri)
+    cond do
+      absolute?(iri)      -> new(iri)
+      not absolute?(base) -> nil
+      true                -> merge(base, iri)
     end
   end
 

test/unit/iri_test.exs

@@ -223,6 +223,12 @@ defmodule RDF.IRITest do
                 IRI.merge(base_iri, relative_iri)
       end
     end
+
+    test "with a relative iri without an absolute base iri" do
+      for relative_iri <- relative_iris(), base_iri <- [nil, "foo"] do
+        assert IRI.absolute(relative_iri, base_iri) == nil
+      end
+    end
   end