From d58516b09fcc8540593c723f9c99339216940966 Mon Sep 17 00:00:00 2001
From: Stephen Paul Weber <singpolyma@singpolyma.net>
Date: Wed, 10 Jul 2019 19:22:33 -0500
Subject: [PATCH] Record projection by expression / type

---
 lib/dhall/ast.rb       | 11 +++++++++++
 lib/dhall/binary.rb    | 10 +++++++++-
 lib/dhall/normalize.rb | 12 ++++++++++++
 lib/dhall/parser.rb    |  2 ++
 lib/dhall/typecheck.rb | 28 ++++++++++++++++++++++++++++
 5 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/lib/dhall/ast.rb b/lib/dhall/ast.rb
index f42690d..0d934e8 100644
--- a/lib/dhall/ast.rb
+++ b/lib/dhall/ast.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
diff --git a/lib/dhall/binary.rb b/lib/dhall/binary.rb
index 2946d35..0c62aab 100644
--- a/lib/dhall/binary.rb
+++ b/lib/dhall/binary.rb
@@ -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
 
diff --git a/lib/dhall/normalize.rb b/lib/dhall/normalize.rb
index d04a64e..7ac48e9 100644
--- a/lib/dhall/normalize.rb
+++ b/lib/dhall/normalize.rb
@@ -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
diff --git a/lib/dhall/parser.rb b/lib/dhall/parser.rb
index 6edf1dc..3013392 100644
--- a/lib/dhall/parser.rb
+++ b/lib/dhall/parser.rb
@@ -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
diff --git a/lib/dhall/typecheck.rb b/lib/dhall/typecheck.rb
index ed212b1..21be8be 100644
--- a/lib/dhall/typecheck.rb
+++ b/lib/dhall/typecheck.rb
@@ -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
 
-- 
2.34.5