2 files changed, 82 insertions(+), 1 deletions(-)

M lib/dhall/normalize.rb
M test/test_normalization.rb

@@ 8,6 8,10 @@ module Dhall
 			map_subexpressions(&:normalize)
 		end
 
+		def shift(amount, name, min_index)
+			map_subexpressions { |expr| expr.shift(amount, name, min_index) }
+		end
+
 		def fusion(*); end
 	end
 


@@ 35,6 39,13 @@ module Dhall
 	end
 
 	class Function
+		def shift(amount, name, min_index)
+			return super unless var == name
+			with(
+				type: type.shift(amount, name, min_index),
+				body: body.shift(amount, name, min_index + 1)
+			)
+		end
 	end
 
 	class Forall; end


@@ 43,6 54,10 @@ module Dhall
 	end
 
 	class Variable
+		def shift(amount, name, min_index)
+			return self if self.name != name || min_index > index
+			with(index: index + amount)
+		end
 	end
 
 	class Operator


@@ 274,6 289,10 @@ module Dhall
 
 	class LetBlock
 		def normalize
+			desugar.normalize
+		end
+
+		def desugar
 			lets.reduce(body) { |inside, let|
 				Application.new(
 					function: Function.new(


@@ 283,7 302,11 @@ module Dhall
 					),
 					arguments: [let.assign]
 				)
-			}.normalize
+			}
+		end
+
+		def shift(amount, name, min_index)
+			desugar.shift(amont, name, min_index)
 		end
 	end
 

@@ 20,4 20,62 @@ class TestParser < Minitest::Test
 			)
 		end
 	end
+
+	def test_shift_1_x_0_x
+		assert_equal(
+			Dhall::Variable.new(name: "x", index: 1),
+			Dhall::Variable.new(name: "x").shift(1, "x", 0)
+		)
+	end
+
+	def test_shift_1_x_1_x
+		assert_equal(
+			Dhall::Variable.new(name: "x", index: 0),
+			Dhall::Variable.new(name: "x").shift(1, "x", 1)
+		)
+	end
+
+	def test_shift_1_x_0_y
+		assert_equal(
+			Dhall::Variable.new(name: "y", index: 0),
+			Dhall::Variable.new(name: "y").shift(1, "x", 0)
+		)
+	end
+
+	def test_shift_neg1_x_0_x1
+		assert_equal(
+			Dhall::Variable.new(name: "x", index: 0),
+			Dhall::Variable.new(name: "x", index: 1).shift(-1, "x", 0)
+		)
+	end
+
+	def test_shift_closed
+		assert_equal(
+			Dhall::Function.new(
+				var: "x",
+				type: Dhall::Variable.new(name: "Type"),
+				body: Dhall::Variable.new(name: "x", index: 0)
+			),
+			Dhall::Function.new(
+				var: "x",
+				type: Dhall::Variable.new(name: "Type"),
+				body: Dhall::Variable.new(name: "x", index: 0)
+			).shift(1, "x", 0)
+		)
+	end
+
+	def test_shift_free
+		assert_equal(
+			Dhall::Function.new(
+				var: "y",
+				type: Dhall::Variable.new(name: "Type"),
+				body: Dhall::Variable.new(name: "x", index: 1)
+			),
+			Dhall::Function.new(
+				var: "y",
+				type: Dhall::Variable.new(name: "Type"),
+				body: Dhall::Variable.new(name: "x", index: 0)
+			).shift(1, "x", 0)
+		)
+	end
 end