Keywords (let) can be used as functions too

lib/keisan/functions/let.rb

@@ -6,8 +6,7 @@ def initialize
       end
 
       def value(ast_function, context = nil)
-        validate_arguments!(ast_function.children.count)
-        assignment(ast_function).value(context)
+        evaluate(ast_function, context)
       end
 
       def evaluate(ast_function, context = nil)
@@ -16,8 +15,7 @@ def evaluate(ast_function, context = nil)
       end
 
       def simplify(ast_function, context = nil)
-        validate_arguments!(ast_function.children.count)
-        assignment(ast_function).simplify(context)
+        evaluate(ast_function, context)
       end
 
       private

lib/keisan/parser.rb

@@ -51,8 +51,15 @@ def parse_multi_line!
 
     def parse_keyword!
       keyword = tokens.first.string
+      arguments = if tokens[1].is_a?(Tokens::Group)
+                    tokens[1].sub_tokens.split {|token| token.is_a?(Tokens::Comma)}.map {|argument_tokens|
+                      Parsing::Argument.new(argument_tokens)
+                    }
+                  else
+                    Parsing::Argument.new(tokens[1..-1])
+                  end
       @components = [
-        Parsing::Function.new(keyword, Parsing::Argument.new(tokens[1..-1]))
+        Parsing::Function.new(keyword, arguments)
       ]
     end
 

spec/keisan/let_spec.rb

@@ -1,17 +1,32 @@
 require "spec_helper"
 
 RSpec.describe Keisan::Functions::Let do
+  let(:calculator) { Keisan::Calculator.new }
+
   it "can be used to assign variables" do
-    calculator = Keisan::Calculator.new
     calculator.evaluate("let x = 4")
     expect(calculator.evaluate("2*x")).to eq 8
   end
 
   it "defines variables locally when in block" do
-    calculator = Keisan::Calculator.new
     calculator.evaluate("let x = 7")
 
     expect(calculator.evaluate("{let x = 11; x*2}")).to eq 22
     expect(calculator.evaluate("x")).to eq 7
   end
+
+  it "raises error for single argument when not assignment" do
+    expect{calculator.evaluate("let x")}.to raise_error(Keisan::Exceptions::InvalidFunctionError)
+  end
+
+  it "works with two arguments for variable assignment" do
+    calculator.evaluate("let(x, (10+20))")
+    expect(calculator.evaluate("x").value).to eq 30
+  end
+
+  it "works in sub-expression" do
+    calculator.evaluate("y = 2*let(x, 3)")
+    expect(calculator.evaluate("x").value).to eq 3
+    expect(calculator.evaluate("y").value).to eq 6
+  end
 end