From 1e9d4f30ec1c82cd7dbf2bbaf6c67d8fc6f0d97e Mon Sep 17 00:00:00 2001
From: Stephen Paul Weber <singpolyma@singpolyma.net>
Date: Fri, 29 Mar 2019 20:06:18 -0500
Subject: [PATCH] Refactor RecordProjection typecheck

---
 lib/dhall/ast.rb       | 12 ++++++++++++
 lib/dhall/typecheck.rb | 18 ++++++++----------
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/lib/dhall/ast.rb b/lib/dhall/ast.rb
index 551ab6f..a64bc9b 100644
--- a/lib/dhall/ast.rb
+++ b/lib/dhall/ast.rb
@@ -443,6 +443,14 @@ module Dhall
 			}.sort])
 		end
 
+		def keys
+			record.keys
+		end
+
+		def slice(keys)
+			RecordType.for(record.select { |k, _| keys.include?(k) })
+		end
+
 		def ==(other)
 			other.respond_to?(:record) && record.to_a == other.record.to_a
 		end
@@ -459,6 +467,10 @@ module Dhall
 	class EmptyRecordType < RecordType
 		include(ValueSemantics.for_attributes {})
 
+		def slice(*)
+			self
+		end
+
 		def record
 			{}
 		end
diff --git a/lib/dhall/typecheck.rb b/lib/dhall/typecheck.rb
index 0193495..fcaab6e 100644
--- a/lib/dhall/typecheck.rb
+++ b/lib/dhall/typecheck.rb
@@ -587,25 +587,23 @@ module Dhall
 		class RecordProjection
 			def initialize(projection)
 				@projection = projection
-				@record = projection.record
+				@record = TypeChecker.for(projection.record)
 				@selectors = projection.selectors
 			end
 
 			def annotate(context)
-				arecord = TypeChecker.for(@record).annotate(context)
+				arecord = @record.annotate(context)
 
-				unless arecord.type.class == Dhall::RecordType
-					raise TypeError, "RecordProjection on #{arecord.type}"
-				end
+				TypeChecker.assert arecord.type.class.name, "Dhall::RecordType",
+				                   "RecordProjection on #{arecord.type}"
 
-				slice = arecord.type.record.select { |k, _| @selectors.include?(k) }
-				if slice.size != @selectors.length
-					raise TypeError, "#{arecord.type} missing one of: #{@selectors}"
-				end
+				slice = arecord.type.slice(@selectors)
+				TypeChecker.assert slice.keys, @selectors,
+				                   "#{arecord.type} missing one of: #{@selectors}"
 
 				Dhall::TypeAnnotation.new(
 					value: @projection.with(record: arecord),
-					type:  Dhall::RecordType.for(slice)
+					type:  slice
 				)
 			end
 		end
-- 
2.34.5