Add some rudimentary type checking

lib/chervil/core.rb

@@ -11,16 +11,37 @@ module Chervil
       !(pairs.map { |pair| pair[0].send(method, pair[1]) }.any?(false))
     end
 
+    def self.display_class_name(cls)
+      case cls.inspect
+      when "Float"
+        "number"
+      when "String"
+        "string"
+      end
+    end
+
+    def self.type_check(args, cls)
+      args_with_wrong_type = args.select { |arg| !arg.is_a?(cls) }
+
+      if args_with_wrong_type.length.zero?
+        nil
+      else
+        expected_type = display_class_name(cls)
+        received_type = display_class_name(args_with_wrong_type.first.class)
+        ::Chervil::Error.new("Expected an argument of type #{expected_type} but got #{received_type}")
+      end
+    end
+
     CORE = {
-      "+" => Proc.new { |args| args.inject(:+) },
-      "-" => Proc.new { |args| args.inject(:-) },
-      "*" => Proc.new { |args| args.inject(:*) },
-      "/" => Proc.new { |args| args.inject(:/) },
+      "+" => Proc.new { |args| type_check(args, Float) || args.inject(:+) },
+      "-" => Proc.new { |args| type_check(args, Float) || args.inject(:-) },
+      "*" => Proc.new { |args| type_check(args, Float) || args.inject(:*) },
+      "/" => Proc.new { |args| type_check(args, Float) || args.inject(:/) },
       "=" => Proc.new { |args| compare_pairs(args, :==) },
-      "<" => Proc.new { |args| compare_pairs(args, :<) },
-      ">" => Proc.new { |args| compare_pairs(args, :>) },
-      "<=" => Proc.new { |args| compare_pairs(args, :<=) },
-      ">=" => Proc.new { |args| compare_pairs(args, :>=) },
+      "<" => Proc.new { |args| type_check(args, Float) || compare_pairs(args, :<) },
+      ">" => Proc.new { |args| type_check(args, Float) || compare_pairs(args, :>) },
+      "<=" => Proc.new { |args| type_check(args, Float) || compare_pairs(args, :<=) },
+      ">=" => Proc.new { |args| type_check(args, Float) || compare_pairs(args, :>=) },
       "and" => Proc.new { |args| !(args.include?(false)) },
       "or" => Proc.new { |args| args.any? { |arg| !!arg == true } },
     }

spec/env_spec.rb

@@ -2,7 +2,7 @@ module Chervil
   RSpec.describe Env do
     it 'has arithmetic from core' do
       env = Env.new
-      expect(env.get('+').call([1, 2], env)).to eq(3)
+      expect(env.get('+').call([1.0, 2.0], env)).to eq(3.0)
     end
 
     it 'inherits from parent env' do

spec/interpreter_spec.rb

@@ -32,5 +32,11 @@ module Chervil
       interpret('(plus-one 5)', env)
       expect(env.get('x')).to eq(nil)
     end
+
+    it 'returns a type error for invalid operation' do
+      expect(interpret('(+ 1 "hello")', Env.new).first).to eq(
+        Error.new("Expected an argument of type number but got string")
+      )
+    end
   end
 end