ahem/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')).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