5 files changed, 62 insertions(+), 1 deletions(-)

M lib/dhall/ast.rb
M lib/dhall/binary.rb
M lib/dhall/normalize.rb
M lib/dhall/parser.rb
M lib/dhall/typecheck.rb

@@ 810,6 810,17 @@ module Dhall
 		end
 	end
 
+	class RecordProjectionByExpression < Expression
+		include(ValueSemantics.for_attributes do
+			record Expression
+			selector Expression
+		end)
+
+		def as_json
+			[10, record.as_json, [selector.as_json]]
+		end
+	end
+
 	class EmptyRecordProjection < Expression
 		include(ValueSemantics.for_attributes do
 			record Expression

@@ 144,7 144,15 @@ module Dhall
 
 	class RecordProjection
 		def self.decode(record, *selectors)
-			self.for(Dhall.decode(record), selectors)
+			record = Dhall.decode(record)
+			if selectors.length == 1 && selectors[0].is_a?(Array)
+				RecordProjectionByExpression.new(
+					record:   record,
+					selector: Dhall.decode(selectors[0][0])
+				)
+			else
+				self.for(record, selectors)
+			end
 		end
 	end
 

@@ 320,6 320,18 @@ module Dhall
 		end
 	end
 
+	class RecordProjectionByExpression
+		def normalize
+			sel = selector.normalize
+
+			if sel.is_a?(RecordType)
+				RecordProjection.for(record, sel.keys).normalize
+			else
+				with(record: record.normalize, selector: sel)
+			end
+		end
+	end
+
 	class EmptyRecordProjection
 		def normalize
 			EmptyRecord.new

@@ 146,6 146,8 @@ module Dhall
 				selectors.reduce(record) do |rec, sels|
 					if sels.is_a?(Array)
 						RecordProjection.for(rec, sels)
+					elsif sels.is_a?(Dhall::Expression)
+						RecordProjectionByExpression.new(record: rec, selector: sels)
 					else
 						RecordSelection.new(record: rec, selector: sels)
 					end

@@ 604,6 604,34 @@ module Dhall
 			end
 		end
 
+		class RecordProjectionByExpression
+			TypeChecker.register self, Dhall::RecordProjectionByExpression
+
+			def initialize(projection)
+				@selector = projection.selector.normalize
+				@projection = projection
+			end
+
+			def annotate(context)
+				TypeChecker.assert @selector, Dhall::RecordType,
+				                   "RecordProjectionByExpression on #{@selector.class}"
+
+				projection = Dhall::RecordProjection.for(
+					@projection.record,
+					@selector.keys
+				)
+
+				TypeChecker.assert_type projection, @selector.normalize,
+				                        "Type doesn't match #{@selector}",
+				                        context: context
+
+				Dhall::TypeAnnotation.new(
+					value: @projection,
+					type:  @selector
+				)
+			end
+		end
+
 		class Enum
 			TypeChecker.register self, Dhall::Enum