Add assignments, make declarations throw if var already exists

lib/ahem/ast.rb

 module AST
+  require 'ahem/ast/assignment'
   require 'ahem/ast/atom'
   require 'ahem/ast/array'
   require 'ahem/ast/binary'

lib/ahem/ast/assignment.rb

+class AST::Assignment
+  attr_reader :name, :value
+
+  def initialize(name, value)
+    @name = name
+    @value = value
+  end
+
+  def ==(other)
+    other.is_a?(AST::Assignment) && other.name == @name && other.value == @value
+  end
+
+  def execute(env)
+    if env.get(@name.name)
+      env.set(@name.name, @value)
+    else
+      raise "Invalid assignment to undeclared variable #{@name.name}"
+    end
+  end
+end

lib/ahem/ast/variable_declaration.rb

   end
 
   def execute(env)
-    env.set(@name.name, @value)
+    if env.data[@name.name].nil?
+      env.set(@name.name, @value)
+    else
+      raise "Invalid declaration of previously declared variable #{@name.name}"
+    end
   end
 end

lib/ahem/parser.rb

   end
 
   def expression
-    binary
+    assignment
   end
 
   private
 
+  def assignment
+    expr = binary
+
+    if @current_token.type == TokenKinds::EQUALS
+      raise 'Invalid left hand side of assignment' unless expr.is_a?(AST::Identifier)
+      eat(TokenKinds::EQUALS)
+      value = expression
+      AST::Assignment.new(expr, value)
+    else
+      expr
+    end
+  end
+
   def binary
     left = comparison
 

spec/ast/assignment_spec.rb

+RSpec.describe AST::Assignment do
+  it 'reassigns an existing variable' do
+    env = Environment.new
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('x'),
+      AST::Number.new(5.0),
+    ).execute(env)
+    AST::Assignment.new(
+      AST::Identifier.new('x'),
+      AST::Number.new(6.0),
+    ).execute(env)
+    expect(env.get('x').execute(env)).to eq(6.0)
+  end
+
+  it 'raises for an undeclared variable' do
+    env = Environment.new
+    expect {
+      AST::Assignment.new(
+        AST::Identifier.new('x'),
+        AST::Number.new(6.0),
+      ).execute(env)
+    }.to raise_error("Undefined variable x")
+
+  end
+end

spec/ast/variable_declaration_spec.rb

+RSpec.describe AST::VariableDeclaration do
+  it 'defines a variable' do
+    env = Environment.new
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('x'),
+      AST::Number.new(5.0),
+    ).execute(env)
+    expect(env.get('x').execute(env)).to eq(5.0)
+  end
+
+  it 'raises if a variable is already defined' do
+    env = Environment.new
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('x'),
+      AST::Number.new(5.0),
+    ).execute(env)
+    expect {
+      AST::VariableDeclaration.new(
+        AST::Identifier.new('x'),
+        AST::Number.new(6.0),
+      ).execute(env)
+    }.to raise_error('Invalid declaration of previously declared variable x')
+  end
+end

spec/parser_spec.rb

       ]
     )
 	end
+
+  it 'parses assignments' do
+    expect(parse('x = 5;')).to eq(
+      [
+        AST::Assignment.new(
+          AST::Identifier.new('x'),
+          AST::Number.new(5.0),
+        )
+      ]
+    )
+  end
 end