12 files changed, 52 insertions(+), 23 deletions(-)

M lib/dhall/ast.rb
M lib/dhall/builtins.rb
M test/normalization/beta/ListHeadEmptyA.dhallb
M test/normalization/beta/ListHeadOneA.dhallb
M test/normalization/beta/ListIndexedEmptyA.dhallb
M test/normalization/beta/ListIndexedOneA.dhallb
M test/normalization/beta/ListLastEmptyA.dhallb
M test/normalization/beta/ListLastOneA.dhallb
M test/normalization/beta/ListLengthEmptyA.dhallb
M test/normalization/beta/ListLengthOneA.dhallb
M test/normalization/beta/ListReverseEmptyA.dhallb
M test/normalization/beta/ListReverseTwoA.dhallb

@@ 323,8 323,7 @@ module Dhall
 
 	class RecordType < Expression
 		include(ValueSemantics.for_attributes do
-			# The nil is not allowed in Dhall
-			record Util::HashOf.new(Either(nil, Expression), min: 1)
+			record Util::HashOf.new(Expression, min: 1)
 		end)
 
 		def deep_merge_type(other)

@@ 220,28 220,40 @@ module Dhall
 		end
 
 		class List_head < Builtin
+			include(ValueSemantics.for_attributes do
+				type Either(nil, Expression), default: nil
+			end)
+
 			def call(arg)
-				if arg.is_a?(List)
-					arg.first
-				else
-					super
+				fill_or_call(arg) do
+					if arg.is_a?(List)
+						arg.first
+					else
+						super
+					end
 				end
 			end
 		end
 
 		class List_indexed < Builtin
+			include(ValueSemantics.for_attributes do
+				type Either(nil, Expression), default: nil
+			end)
+
 			def call(arg)
-				if arg.is_a?(List)
-					_call(arg)
-				else
-					super
+				fill_or_call(arg) do
+					if arg.is_a?(List)
+						_call(arg)
+					else
+						super
+					end
 				end
 			end
 
 			protected
 
 			def _call(arg)
-				arg.map(type: indexed_type(arg.type)) do |x, idx|
+				arg.map(type: indexed_type(type)) do |x, idx|
 					Record.new(
 						record: {
 							"index" => Natural.new(value: idx),


@@ 262,31 274,49 @@ module Dhall
 		end
 
 		class List_last < Builtin
+			include(ValueSemantics.for_attributes do
+				type Either(nil, Expression), default: nil
+			end)
+
 			def call(arg)
-				if arg.is_a?(List)
-					arg.last
-				else
-					super
+				fill_or_call(arg) do
+					if arg.is_a?(List)
+						arg.last
+					else
+						super
+					end
 				end
 			end
 		end
 
 		class List_length < Builtin
+			include(ValueSemantics.for_attributes do
+				type Either(nil, Expression), default: nil
+			end)
+
 			def call(arg)
-				if arg.is_a?(List)
-					Natural.new(value: arg.length)
-				else
-					super
+				fill_or_call(arg) do
+					if arg.is_a?(List)
+						Natural.new(value: arg.length)
+					else
+						super
+					end
 				end
 			end
 		end
 
 		class List_reverse < Builtin
+			include(ValueSemantics.for_attributes do
+				type Either(nil, Expression), default: nil
+			end)
+
 			def call(arg)
-				if arg.is_a?(List)
-					arg.reverse
-				else
-					super
+				fill_or_call(arg) do
+					if arg.is_a?(List)
+						arg.reverse
+					else
+						super
+					end
 				end
 			end
 		end