Implement car and cdr

lib/chervil/core.rb

@@ -63,6 +63,36 @@ module Chervil
           error
         end
       end,
+      "car" => Proc.new do |args|
+        error = arity_check(args, 1)
+        if error.nil?
+          error = type_check(args, Array)
+          if error.nil?
+            if args.first.empty?
+              Error.new("`car` expects a non-empty list")
+            else
+              args.first.first
+            end
+          else
+            error
+          end
+        else
+          error
+        end
+      end,
+      "cdr" => Proc.new do |args|
+        error = arity_check(args, 1)
+        if error.nil?
+          error = type_check(args, Array)
+          if error.nil?
+            args.first[1..-1]
+          else
+            error
+          end
+        else
+          error
+        end
+      end
     }
   end
 end

spec/core_spec.rb

@@ -37,5 +37,25 @@ module Chervil
         Error.new("Expected 1 argument but received 2")
       )
     end
+
+    it 'can do car and cdr' do
+      expect(Core::CORE['car'].call([[1, 2, 3]])).to eq(1)
+      expect(Core::CORE['cdr'].call([[1, 2, 3]])).to eq([2, 3])
+      expect(Core::CORE['car'].call([1.0])).to eq(
+        Error.new("Expected an argument of type list but got number")
+      )
+      expect(Core::CORE['cdr'].call([1.0])).to eq(
+        Error.new("Expected an argument of type list but got number")
+      )
+      expect(Core::CORE['car'].call([[1, 2, 3], [4, 5, 6]])).to eq(
+        Error.new("Expected 1 argument but received 2")
+      )
+      expect(Core::CORE['cdr'].call([[1, 2, 3], [4, 5, 6]])).to eq(
+        Error.new("Expected 1 argument but received 2")
+      )
+      expect(Core::CORE['car'].call([Array.new])).to eq(
+        Error.new("`car` expects a non-empty list")
+      )
+    end
   end
 end