ahem/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