ruby - 创建一个绑定(bind)，其唯一变量是哈希中的键

Question

我知道我可以使用 ostruct 从哈希创建绑定(bind)，如下所示:

require 'ostruct'

not_in_locals = "we don't want this"
locals = {a: 1, b: 2}
my_binding = OpenStruct.new(locals).instance_eval { binding }
p my_binding
p eval("a", my_binding) #good, we want this
p eval("b", my_binding) #good, we want this
p eval("not_in_locals", my_binding) #bad, don't want it to access this, but it can

您可以在此处看到输出，这证实了代码中的注释:https://eval.in/132925

如您所见，问题在于 Binding 还绑定(bind)了局部上下文中的变量，这些变量不在哈希中。我想要一种从 Hash 创建 Binding 对象的方法，除了来自该 Hash 的键之外，这些对象不绑定(bind)任何东西。

Answer 1

你可以尝试这样的事情。它绕过对辅助类方法调用的直接 eval() 调用。辅助类方法称为 evaluate()，但它仅适用于非常简单的值(目前仅测试过字符串和整数)并且依赖于 inspect()。

但是如果你正在处理的值的类型是提前知道的，你可以修改它来工作。

class HashBinding
  def initialize(hash)
    @hash = hash
  end

  def evaluate(str)
    binding_code = @hash.to_a.map do |k,v|
      "#{k} = #{v.inspect};"
    end.join(" ")
    eval "#{binding_code}; #{str}"
  end
end

not_in_hash = 'I am not in the hash'
hash = { :foo => 'foo value', :baz => 42}
hash_binding = HashBinding.new(hash)

hash_binding.evaluate('puts "foo = #{foo}"')
hash_binding.evaluate('puts "baz = #{baz}"')
hash_binding.evaluate('puts "not_in_hash = #{not_in_hash}"')

输出是:

ruby binding_of_hash.rb
foo = foo value
baz = 42
binding_of_hash.rb:10:in `eval': undefined local variable or method `not_in_hash' for #<HashBinding:0x007fcc0b9ec1e8> (NameError)
    from binding_of_hash.rb:10:in `eval'
    from binding_of_hash.rb:10:in `evaluate'
    from binding_of_hash.rb:20:in `<main>'