Return error for unbound variable/function

lib/chervil.rb

@@ -1,6 +1,7 @@
 require 'chervil/ast'
 require 'chervil/core'
 require 'chervil/env'
+require 'chervil/error'
 require 'chervil/interpreter'
 require 'chervil/lexer'
 require 'chervil/parser'

lib/chervil/ast/application.rb

@@ -15,7 +15,12 @@ module Chervil::AST
     def evaluate(env)
       if @expr.class == Identifier
         function = env.get(@expr.name)
-        function.call(@arguments.map { |arg| arg.evaluate(env) })
+
+        if function.nil?
+          ::Chervil::Error.new("Unbound variable #{@expr.name}")
+        else
+          function.call(@arguments.map { |arg| arg.evaluate(env) })
+        end
       end
     end
   end

lib/chervil/ast/identifier.rb

@@ -11,7 +11,12 @@ module Chervil::AST
     end
 
     def evaluate(env)
-      env.get(@name).evaluate(env)
+      value = env.get(@name)
+      if value.nil?
+        ::Chervil::Error.new("unbound variable #{@name}")
+      else
+        value.evaluate(env)
+      end
     end
   end
 end

lib/chervil/error.rb

@@ -0,0 +1,17 @@
+module Chervil
+  class Error
+    attr_reader :message
+
+    def initialize(message)
+      @message = message
+    end
+
+    def ==(other)
+      @message == other.message
+    end
+
+    def to_s
+      "Error: #{@message}"
+    end
+  end
+end

spec/ast/application_spec.rb

@@ -30,5 +30,12 @@ module Chervil
           .evaluate(env)
       ).to eq(10.0)
     end
+
+    it 'returns an error if the function is not defined' do
+      env = Env.new
+      expect(AST::Application.new(AST::Identifier.new("x"), []).evaluate(env)).to eq(
+        Error.new("Unbound variable x")
+      )
+    end
   end
 end

spec/ast/identifier_spec.rb

@@ -8,5 +8,11 @@ module Chervil
       definition.evaluate(env)
       expect(identifier.evaluate(env)).to eq(5.0)
     end
+
+    it 'returns an error for an unbound identifier' do
+      env = Env.new
+      identifier = AST::Identifier.new("x")
+      expect(identifier.evaluate(env)).to eq(Error.new("unbound variable x"))
+    end
   end
 end